have held, albeit mostly (but not exclusively) in the criminal context, that an error cannot be found harmless if it “precludes or impairs the presentation of [a defendant’s] sole means of defense.” United States v. Carter, 491 F.2d 625, 630 (5th Cir.1974); see also United States v. Peak, 856 F.2d 825, 834-35 (7th Cir.1988); United States v. Harris, 733 F.2d 994, 1005 (2d Cir.1984); accord United States v. Evans, 728 F.3d 953, 967 (9th Cir.2013) (concluding that trial judge’s erroneous exclusion of the “central piece of evidence” for defendant’s “main defense” and which went to the “very heart” of the dispute could not be harmless, notwithstanding the “overwhelming volume and substance of the government’s evidence”); Ashcraft & Gerel v. Coady, 244 F.3d 948, 949, 954 (D.C.Cir.2001) (<HOLDING>). The error was not only not harmless, but, it
holding that the defense may present the entire conversation where it goes to the heart of the  defense holding that trial error in a civil case which went to the very heart of the defendants defense and was central to his defense could not be deemed harmless in the absence of any steps by the district court to mitigate the effects of the error holding that the district courts error in calculating the amount of drugs at issue was harmless because the error had no impact on the defendants sentence holding that defendants failure to assert the defense in any pretrial motions did not waive defendants limitations defense because the assertion of a limitations defense in the answer preserved defendants right to raise the defense both during the first trial and before the second holding that any error was harmless and thus not plain error