300 - Leonidas, Leonidas, Leonidas. Would it have killed you to give Ephialtes a shot at glory? You could've sent him out ahead of your phalanx, have him charge into the Persians and do his thing. He'd most likely get cut down, but I'm sure he'd take a few Persians with him - just like a real Spartan would. That way, he'd never have betrayed you to Xerxes, you and the Spartans wouldn't have been outflanked, and we'd get some more battle scenes.
Star Wars: Revenge of the Sith - Anakin, why the FUCK didn't you listen to Mace? You wait in the Chambers, Mace destroys Palpatine, you eventually become a Jedi Master, and get to spend the rest of your life drilling Amidala and playing with your kids on Naboo, teaching them the ways of the Force. Putz.