Factual error: Near the end of the movie, Agent Terrence asks Mitch why he didn't release the recording to the media. Mitch replies that it would have been against the law. This is actually incorrect. Tennessee, where the movie is based, is a one-party consent state. Which means only one party needs to give consent to a conversation being recorded. That one party is Mitch McDeere.
Character mistake: At the end, when Mitch tells Agent Tarrance that the government can convict the law firm on mail fraud and racketeering, he says he got the idea while he was studying for the bar exam. He actually got the idea from a client earlier in the film when the client was complaining about being over-billed and tells Mitch that every time a bill is mailed through USPS, the firm is committing mail fraud which is a federal offense, punishable by fines and prison.
Suggested correction: This is not wholly correct. Mulholland (the client) planted the seed in Mitch's head, but Mitch realised instantly that it was a federal offense, presumably because he had studied it.