So... You've just emerged from the comforting, air-conditioned darkness of the movie theatre, and you're feeling inspired. Finally! A movie that's shown you what you should be doing with your life. You're going to be a lawyer, and find the evidence the cops were too dumb to pick up on, shocking the courtroom in the third act of the trial. Or maybe you're going to go home to build a suit of weaponized armor and fly around the world, fighting villains. Or, you're determined to drive in a manner that is both faster and angrier than usual, pulling off lucrative heists and then retiring to an obscure country, far from the reach of The Law.
Well, guess what? Legal systems in real life are NOTHING like the movies. You can't do any of these things, and most of them will end with you moving into a room with a view through iron bars. For, as you will soon see, cinema has a very warped sense of legality in this world. There are countless ways real life courtrooms are different from movies and of all the things Hollywood embellishes, court cases and the legal system in general might get the worst treatment.
Surprise Evidence Cannot Be Presented During Trial
Almost every courtroom drama climaxes with a scene in which the upstart, underdog lawyer turns the tide of the trial with case-breaking evidence no one knew existed. It's very exciting to see, but real law cares nothing for entertainment value. In reality, all evidence must be presented to the opposing side during the discovery phase before trial.
The intense, edge-of-your-seat courtroom drama may be the biggest fallacy of all. Even the most violent, serial killer prosecutions are leaden with the minutiae of unknowing witnesses, innumerable repetitions of well-known facts, exhaustive examination of timing, paperwork, credentials, ad nauseam... If you think being a lawyer is fun, ask some lawyers about it.
According to the rules invented by Hollywood, if a criminal asks an undercover officer whether he is a cop or not, said officer MUST respond with the truth, or else it's entrapment. This is nonsense. Entrapment occurs when someone in law enforcement does something to cause a person to commit a criminal act they wouldn't have committed otherwise. If you're at the stage where you feel the need to ask if someone's a cop before you continue, your criminal intentions are already obvious.
Federal Agents Don't Have the Authority to Promise Immunity
There are myriad examples, including The Rock's Hobbs granting the Fast and Furious gang varying degrees of immunity for their ludicrous crimes at different points throughout the series. Federal agents can't just do this on the fly. It takes reams of legal red tape to secure immunity from prosecution for federal crimes. Also, deputizing a bunch of untrained felons and giving them weapons to help you fight even badder bad guys? Pretty sure that's not a thing.