What is the test for an error in testimony in a civil case?

The parties, not surprisingly, differ on the applicable standard for evaluating the error. According to defendant, the error was so serious that it rendered the trial fundamentally unfair and was not harmless beyond a reasonable doubt. (Chapman v. California (1967) 386 U.S. 18, 24 [17 L.Ed.2d 705].) In his view, the error undermined his only defense to the firearms and ammunition charges alleged in counts 1 and 2--that he did not exercise dominion and control over the guns and ammunition. He also claims the evidence damaged his credibility because the statements could be construed as admissions that he in fact exercised dominion and control over the guns and he testified to the contrary.