Even one mistake that the opponent can establish will result in the mistaken attorney losing the confidence and trust of the jury for the balance of that trial and perhaps the respect of the judge for the balance of her career. I've taken a similar approach to authenticity in my novels. Several chapters are set in Vietnam. Now, I was in the military police, but never went overseas. Accordingly, I read 12 books on the war, from Stanley Karnow's comprehensive non-fiction work to Mark Baker's oral histories, just to be sure I had enough background to write those chapters.
Authenticity is important in little details, too.
One of the chapters requires him to go to a local jail to interview the criminal defendant. While in civilian law enforcement, I'd worked one jail and visited others, but not that particular jail. I arranged through the Sheriff's Office to get a tour of the facility, a helpful deputy walking me through the exact sequence a private investigator would have to follow in order to see a client. Such attention to authenticity wins the writer many readers that a parallel inattention would lose for him. A third way that being a trial attorney helps me as a writer is in generating story ideas.