possibility of a substantial change or any change at all, or an inspection will disclose no obvious alteration or modification of its condition from the one that existed at the time of manufacture, sale, or delivery. Therefore, while the plaintiff may not necessarily come forward affirmatively with proof that the condition is unchanged, the defendant cannot, in good faith, assert that there is anything amiss. (Footnote omitted.) Given the ad hoc applicability of “substantial change,” it is not surprising that different views have emerged as to the proper distribution of the burden of proof on the issue. Some courts require the plaintiff to affirmatively prove the absence of any substantial change. See Waggoner v. Mercedes Benz of N. Am., Inc., 879 S.W.2d 692, 695-96 (Mo.Ct.App.1994) (<HOLDING>); Jasinski v. Ford Motor Co., 824 S.W.2d 454,
holding that district court did not abuse its discretion in refusing to permit a design defect products liability expert to submit an untimely addendum stating a new claim of failure to warn after defense expert refuted the basis of original design defect opinion holding plaintiff with inherently defective heart valve failed to state a claim unless the valve malfunctioned holding that plaintiffs did not need to identify the specific defective lawn mower to sustain their claim of a design defect holding that complaint alleging design defect likely to cause damage failed to state a claim holding that plaintiffs failure to prove that the subject air valve had not been altered in some way invalidated their design defect claim