Continuity mistake: When they jump in the pool, they drop the guitar. When they jump out their guitar is no longer in the pool.
Continuity mistake: When Mama Imelda is singing on the stage and in the shot where she runs towards Miguel and Héctor, you can see several security guards on the stage attempting to apprehend her. However in the next shot that shows Ernesto grabbing her hand, there are no security guards visible. Three of them reappear briefly (the shot lasts about a second but you can see them) but they disappear for the rest of the scene. (01:23:05)
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.