Continuity mistake: Notice Marty's hair in the scene when he's playing "Johnny B. Goode." It's quite larger than when he says, some seconds later, "I guess that you're not ready for it yet." That's because when he says that, it's a recycled shot from the original BTTF, whereas most shots showing he's playing the guitar are newly made for BTTF2. However, Marty's hair is considerably longer in the new shots.
Continuity mistake: Early in the film Bud throws his wedding ring down the latrine and then retrieves it with his right hand so it is covered with the blue disinfectant. In a (much) later scene, Bud has a blue right hand, despite not having had one in the intervening period. (01:44:48)
Visible crew/equipment: When the girl falls to the ground in the warehouse, the safety mat she falls on is visible.
Revealing mistake: When the main guy is fighting the main girl in his room, the scene changes angles and you can CLEARLY see that her stunt double is a man!
Continuity mistake: When the devil sets Toxie on fire, there's a group of five people standing there with empty buckets. But in the next shot the buckets are filled with water.
Continuity mistake: When Wallace races out to get the crackers Gromit is seen spinning in his chair, to the right of the screen is the handbrake in the down position. When the ship blasts off Gromit realises he's left it on and puts it from the up position to the down position. (00:08:30 - 00:09:20)Sanguis