ALVIN B. RUBIN, Circuit Judge:
An unfortunate woman lost the tips of two fingers when a bacon-slicing machine she was cleaning unexpectedly began to operate. Her misfortune was compounded when the jury in her tort suit against the manufacturer of the machine found for the defendant. She now appeals, alleging that the jury’s verdict was insufficiently supported by evidence and that the district court made five erroneous evidentiary rulings, the most important of which excluded evidence of a subsequent design change that the woman asserts should have been admitted for purposes of impeachment. We hold that the district court’s evidentiary rulings were within its discretion and that there was evidence sufficient to support the jury’s verdict.
I.
On October 5, 1983, Brenda Hardy lost the tips of two fingers while cleaning the bacon- and paper-slicing machine at which she worked. The power button had been switched off and the safety cage was raised, but the cutter nonetheless began to revolve. Invoking diversity jurisdiction, and relying on Mississippi law, Hardy sued Chemetron Corporation, the manufacturer of the machine, alleging that the machine was defective and unreasonably dangerous.
II.
Hardy first argues that the district court should have directed a verdict in her favor. The sheer began to cycle when turned off and while its safety cage was raised, she points out, and thus, “did not meet reasonable expectations of safety, and was, as a matter of law, ‘unreasonably dangerous.’ "
In order to recover under Mississippi law, Hardy was required to prove: “(1) that the plaintiff was injured by the product, (2) that the injury resulted from a defect in the product which rendered it unreasonably dangerous, and (3) that the defect existed at the time it left the hands of the manufacturer.” Even if Chemetron’s machine was unreasonably dangerous at the time Hardy was injured, therefore, she had the burden of demonstrating that the defect existed when the machine left Chemetron’s control.
Chemetron presented credible evidence that the unsafe condition of the machine developed because of poor maintenance by Mid-South, Hardy’s employer. Cheme-tron’s expert witness, Jeffrey Bookwalter, testified that the accident was caused by a short-circuit that resulted from the failure of Mid-South to maintain the wiring of the machine. Because the jury could have believed this evidence, the district court properly refused to direct a verdict, and we cannot conclude that the jury’s verdict was against the great weight of the evidence.
III.
Hardy next objects that Bookwalter’s testimony should have been excluded because his response to interrogatories “contains not a hint that the expert witness was of the opinion that the accident was caused by a short circuit.” Chemetron responds by quoting from its reply to interrogatories: “Mr. Bookwalter will testify that the subject machine was poorly maintained and that this could have caused or contributed to the accident.” Although this statement was not specific, it sufficed to give Hardy fair warning of the expert’s expected testimony. While Bookwalter appears to have become convinced of the theory that a short-circuit had occurred only after listening to trial testimony, and stated the theory for the first time at trial, this did not mandate its exclusion. Moreover, the district court had discretion to refuse to sanction Chemetron by excluding Bookwal-ter’s testimony even if, on balance, Cheme-tron should have supplemented its responses to interrogatories.
IV.
Hardy asserts that the district court also erred in several evidentiary rulings. In reviewing these rulings we recognize that a trial court must make hundreds of evidentiary rulings in the course of a trial and often must decide whether to admit or exclude evidence almost instantaneously. No trial judge can be, or is expected to be, infallible in making these decisions. The trial judge moreover has the feel of the trial, sensing its tempo, the effect of evidence, and the likely probative value of proffered testimony. Acknowledging both our respect for the local judge’s superior knowledge of the trial scene and the importance of enabling the trial judge to keep the trial on course, we accord considerable deference to the trial judge’s evidentiary rulings. In assessing Hardy’s claims, therefore, we apply two well-settled principles of law. The first is stated in Federal Rule of Evidence 103(a): “Error may not be predicated upon a ruling which admits or excludes evidence unless a substantial right of the party is affected,” and the second is that, as a general precept, we overturn an evidentiary ruling and, in consequence, reverse judgments and grant new trials, only if the ruling was so erroneous as to constitute an abuse of discretion.
Hardy first challenges the trial court’s refusal to admit evidence concerning a prior accident that had occurred with a machine that she claims was similar to the slicer that injured her. The court excluded the evidence because Hardy had failed to prove that the two accidents were substantially similar, but Hardy claims that she was unable to prove similarity only because Chemetron informed her of the prior accident a mere eight days before trial. Hardy did, however, over Cheme-tron’s objection, obtain an admission from Chemetron’s expert that another accident had occurred before the one that injured her. She thus was not totally unable to present evidence concerning the matter.
Chemetron had discovered the prior accident only after filing its initial replies to interrogatories, and does not appear to have been in bad faith in failing to supplement its answers earlier. If eight days did not provide sufficient time for Hardy to investigate the accident, she should have requested that trial be postponed. Instead, she declared herself ready for trial, and cannot now complain that the trial judge should have ignored her total lack of evidence on the crucial question of substantial similarity.
V.
Hardy next claims that the district court should not have excluded several admissions allegedly made by Chemetron, including a statement by a claims adjuster that Hardy’s machine had “an electrical problem.” Because Hardy identifies only two specific admissions that she asserts were improperly excluded, this court can consider only those two.
First, the purported admission that the slicer had an electrical problem was made by a claims adjuster who was an agent of Chemetron’s insurer, but not of Chemetron itself. Moreover, the adjuster’s statement was arguably so vague as to be irrelevant and certainly too vague for its exclusion to merit a retrial of this case. Although the jury did not learn that the adjuster wrote, without explanation, that “the problem with the machine was an electrical problem,” we cannot believe that the jury’s ignorance of this fact, even if it resulted from an erroneous evidentiary ruling, affected any substantial right.
The second purported admission that Hardy claims should not have been excluded was a statement in a letter by Steven Newell, defendant’s engineer, that engineering changes disabling two motors as well as the clutch in the sheer would prevent accidental actuation of the machine. Once again, the district court did not err. In context, Newell’s statement clearly referred to a subsequent design change, evidence of which was inadmissible. Moreover, at trial Chemetron admitted the truth of Newell’s statement that redesigning the machine would help prevent accidental injuries, and the district court allowed Newell to testify orally that rewiring the sheer would have made it safer. We can find no abuse of discretion here.
VI.
Hardy’s next argument reflects a basic misunderstanding of the law of evidence. The district court excluded a statement by Andrew Mumyak that only one of Chemetron’s machines was wired as Hardy’s was wired because Murnyak had submitted an affidavit asserting that this remark had referred to an aspect of the wiring not relevant to this case. Hardy claims that the jury might have disbelieved Mumyak’s affidavit concerning the context of his statement, that questions of credibility should be left to the jury, and consequently, that Murnyak’s statement should have been admitted.
Hardy’s argument must be rejected. “When the relevancy of evidence depends upon the fulfillment of a condition of fact,” Federal Rule of Evidence 104(b) provides, “the court shall admit it upon ... the introduction of evidence sufficient to support a finding of the fulfillment of the condition.” Thus the jury is called upon to judge the credibility of evidence whose relevance is conditioned on fact only after the party offering the evidence has made a threshold showing of relevance to the court. In this case, no credible evidence was presented to support Hardy's claim that Murnyak’s statement concerned the wiring fault that allegedly caused her injury. Hardy’s brief mentions no such evidence, and Michael Stevens, the man to whom the statement was made, conceded that he would not contest Murnyak’s affidavit that his statement did not concern the alleged wiring defect at issue. Moreover, Murnyak’s statement was too tangentially related to the issues in dispute at trial for its exclusion, even if erroneous, to have affected any substantial right. Excluding Murnyak’s statement was not therefore error.
VII.
Finally, Hardy asserts that the district court should have admitted evidence of a subsequent design change. After Hardy’s accident, and apparently to avoid a recurrence of such an episode, Chemetron rewired its slicers so that the safety features of the machine would disable its motors as well as its clutch. In order to encourage design changes after accidents have occurred, the Federal Rules of Evidence provide, “evidence of ... subsequent measures is not admissible to prove negligence or culpable conduct in connection with the event.” This rule applies in strict liability as well as in negligence actions. The Rules also provide an exception, however: “This rule does not require the exclusion of evidence of subsequent measures when offered for another purpose, such as proving ownership, control, or feasibility of precautionary measures, if controverted, or impeachment.”
Evidence of Chemetron’s rewiring should have been admitted, Hardy claims, “to impeach Defendant’s trial position that the cause of this accident was not the way the machine was wired, and also ... Defendant’s claim that, after all, this machine had operated properly after the accident.” Thus Hardy first maintains that she should have been allowed to impeach Chemetron’s “trial position” that negligent wiring had not caused her injury, which, minus a double negative, amounts to saying that she should have been allowed to adduce evidence of the rewiring to prove Chemetron’s negligence. This is precisely what Rule 407 was designed to prevent. As the Seventh Circuit has explained:
This exception must be applied with care, since “any evidence of subsequent remedial measures might be thought to contradict and so in a sense impeach [a party’s] testimony that he was using due care at the time of the accident ... [I]f this counted as ‘impeachment’ the exception would swallow the rule.”
This circuit has recognized that risk and held that the trial judge should guard against the improper admission of evidence to prove prior negligence under the guise of impeachment.
Evidence of subsequent measures is no more admissible to rebut a claim of non-negligence than it is to prove negligence directly. Hardy’s argument is but a semantic manipulation, and must therefore be rejected. Even if the decision to admit or exclude the evidence of the design change was a close call, moreover, we do not believe that excluding it was beyond the bounds of the district court’s discretion.
Muzyka v. Remington Arms Co., which Hardy claims “controls,” is in fact inappo-site. In Remington Arms, the defendant stressed the safety and superiority of its products at such length that a panel of this circuit held, over Judge Gee’s dissent, that evidence of a subsequent design change should have been admitted “in impeachment of the experts who spoke in ... superlatives,” experts who had described a rifle that the plaintiff claimed had misfired as “the premier rifle, the best and the safest rifle of its kind on the market.” Chemetron’s experts did not testify in such terms, and Chemetron conceded the feasibility of the design change they had adopted after Hardy’s accident. There was no risk that the jury in this case would have been misled by a defendant’s extravagant boasts. Consequently, Remington Arms does not mandate reversal here.
Hardy further maintains, as quoted above, that evidence of the subsequent design change should have been admitted to impeach Chemetron’s claim that the machine she had operated functioned without incident after her injury. This claim, she asserts, was made in the testimony of Ella Mae McIntosh. At first glance, this appears to be Hardy’s most compelling argument; the facts, however, do not support it. McIntosh testified that she had operated Hardy’s machine without incident after Hardy’s injury, but only on the day the injury occurred, which was three weeks before any rewiring took place. Thus McIntosh made no misleading claims about the subsequent functioning of the machine, and evidence of the subsequent design change would not have impeached her testimony.
In her brief, Hardy points to no other evidence that the subsequent design change might have been used to impeach. At oral argument, however, her counsel extended her impeachment theory in two respects. First, he suggested that evidence of the subsequent change should have been admitted to rebut testimony that the design Hardy advocated was impractical. Chemetron, however, provided ample evidence that it was not challenging the practicality of rewiring: it admitted in a deposition that was read to the jury that the wiring change was both technically and economically feasible, and further stated that it did not know of any disadvantages of rewiring. The sole testimony on the subject at trial was that it was “more practical” to wire the machine in the way it was originally wired to prevent “wear and tear.” Chemetron’s expert did not testify or state, however, that the alternative design was impractical. The district court therefore did not abuse its discretion in refusing to admit evidence of the subsequent design change to rebut testimony concerning the impracticality of rewiring.
Second, Hardy’s counsel asserted that the evidence of the subsequent change should have been admitted to rebut testimony that the design change would not have prevented the accident. The fact that the change was made, however, in no way demonstrates whether the change would or would not have prevented the accident. The district court therefore did not err by excluding evidence of the subsequent design change, and at the very least, was well within the bounds of its discretion in refusing to admit it.
Although other theories might now be developed to support Hardy’s impeachment claim, only those we have addressed were advanced in either the briefs or the oral argument. We cannot and do not overturn district court judgments on the basis of arguments that counsel might have, but did not, make. Arguments conceived for the first time after both actual trial and appellate briefing are but l’esprit de I’escalier.
VIII.
For the foregoing reasons, the judgment in this case is AFFIRMED.
. Early-Gary, Inc. v. Walters, 294 So.2d 181, 186 (Miss.1974).
. See Murphy v. Magnolia Electric Power Ass’n, 639 F.2d 232, 234-35 (5th Cir.1981).
. See, e.g., McGonigal v. Gearhart Indus., Inc., 851 F.2d 774, 777 (5th Cir.1988); Stitt Spark Plug Co. v. Champion Spark Plug Co., 840 F.2d 1253, 1258 (5th Cir.), cert. denied, — U.S. -, 109 S.Ct. 224, 102 L.Ed.2d 214 (1988).
. Fed.R.Evid. 407.
. Grenada Steel Indus, v. Alabama Oxygen Co., 695 F.2d 883 (5th Cir.1983).
.Fed.R.Evid. 407.
. Public Service Co. of Indiana v. Bath Iron Works Corp., 773 F.2d 783, 792 (7th Cir.1985) (quoting Flaminio v. Honda Motor Co., 733 F.2d 463, 468 (7th Cir.1984)).
. Bickerstaff v. South Central Bell Tel. Co., 676 F.2d 163, 168 (5th Cir.1982) (quoting 10 Moore’s Federal Practice, § 407.04 at p. IV-159 (2d ed. 1981».
. 774 F.2d 1309 (5th Cir.1985).
. Id. at 1313 (original emphasis).