Expert witnesses can make a case—or they can wreck it. Or both.

Take the testimony of an expert who gave a federal jury in Chicago reason to award power tool maker Black & Decker $54 million in a trademark and trade dress suit in 2015. That is, until the judge this week tossed the verdict based on the expert's “flawed testimony” which “unfairly influenced the jury's verdict.”

U.S. District Judge Robert Dow of the Northern District of Illinois ordered a new trial, never mind (ahem) that he was the one who greenlighted expert James Berger in the first place. And even though the defendants in 2015 pointed out almost everything that was wrong with Berger's testimony in an unsuccessful bid to disqualify him.