Continuity mistake: At the beginning of the second movie when Marty is checking out the new car, he audibly bangs the car with his fist right below the window frame. In the version of the same scene at the end of the first movie, Marty makes a similar hand motion but does not bang the car, and there is no noise.
Continuity mistake: When Biff, in the alternative 1985, presents the record book to Marty, he puts it in a safe behind a portrait, and then he closes it. Biff continues speaking and you can see that the portrait is slowly opening again. Cut to Marty, then back to Biff, and you can see that now the portrait is completely closed.
Continuity mistake: In the Cafe 80's sequence we see little Elijah Wood and his friend investigating the Wild Gunman game. One of them asks "How do you play this Thing?" while Marty is several feet away, but Marty answers immediately while standing right next to it. So he has managed to walk about 15 feet or so in a fraction of a second.
Continuity mistake: When the DeLorean first lands in the alley in 2015, there is a shot of the back of the car as it lands. The large vents that extend out of Doc's modifications to the car are the wrong size. In most shots, they extend over the car's tail lights, however in this shot they stop above the lights. According to commentary this is a fiberglass scale model of the DeLorean used to film flying scenes, since it is lighter than the actual vehicle, so apparently the model was not quite identical to the real DeLorean.
Continuity mistake: When Marvin Berry leaves the stage to call his cousin Chuck Berry, in Back to the Future II he stops and walks around a ladder that Mary climbed to get on the catwalk over the stage. Watch Back to the Future I and when he is talking on the phone there is no ladder behind him. In fact, that whole scene is different. The desk is different along with the phone and Marvin has his back to the stage in II whereas in I he is facing the stage. (01:26:09)
Continuity mistake: In the original BTTF the photo used to determine if Marty would exist in 1985 (the one with his brother and sister) is a wallet size photo. Yet when Marty in BTTF II (with the leather jacket) sees the other Marty from BTTF look at that photo and rush back to the dance, it's a regular size photo. (01:22:27)
Continuity mistake: In the first BTTF at the scene were George Mcfly knocks out Biff and takes Lorraine, Marty sees them while he is standing in front of a couple. When he goes through the couple to get to the band performing at the dance, the woman in the couple makes a noise and gets pushed aside as he tries to get through them. In BTTF 2 when the Marty back from 1985 is watching George Mcfly knock Biff out before he steals the Almanac, the Marty from 1955 stands in front of the couple again, but this time the couple leaves a gap for him to go through, and when he goes through the woman in the couple, she makes no noise and doesn't get pushed. It was as if she was expecting it that time. As well, the couple isn't the same.
Continuity mistake: In the high school dance scene, the guitar amplifier is standing on the floor. In the same scene in Back To The Future 1 it's raised up on some kind of box.