Other mistake: When Miguel is singing "Un poco loco" on stage, he goes out of range of the mic, but his voice is amplified.
Other mistake: When Hector is singing to his forgotten friend, he takes a drink and the drink doesn't exit his body...where does the liquid go?
Suggested correction: It is safe to say this film chose for the laws in the land of the dead to not be followed the same way they are with the living. While it is scientifically correct that the liquid should go somewhere; if we get that technical the film would be a mistake in its entirety as the skeletons should not be able to talk, walk, or drink at all without muscles and other organs. When the entire nature of the film is fantasy, artistic license allows suspension of the laws of real life science.
Other mistake: Miguel was drowning in the swimming pool in De La Cruz's party but when he was thrown in the pit he managed to swim out of it.