Other mistake: In the scene where Biff confronts Marty outside the swinging doors while Marty is watching himself talk to his 1955 Mom and Dad, you can see that Crispin Glover (George McFly) has been replaced with Jeffrey Weissman. There are other scenes where you can tell it isn't him, but this is the most prominent one. Glover wasn't in the 2nd movie due to him and the producers not agreeing to a movie contract. All the scenes you see him clearly were taken from the first film. (i.e. where Marty is looking at him through the binoculars at the dance). (01:27:59)JamesP
Revealing mistake: When Marty is burning the almanac, during the lightening strike, the shadow of the post holding the hover-board off the ground is visible. - watch the hoverboard when the scene is illuminated by lightning strikes. The post holding up the hoverboard is also clearly visible when they pan around Marty as he is holding the burned rope from the Delorean.BocaDavie
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 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)
Revealing mistake: When Griff and his gang are chasing Marty on the hoverboards in a few shots after Marty is hanging off the back of the Jeep, when the shot shows the gang catching up, the Jeep goes through some wet places in the street, showing the camera car's tire tracks. They're not the Jeep's because the tire tracks are double-wheeled and the Jeep was not turning in that shot.
Visible crew/equipment: When old Biff is driving young Biff's car to Biff's house to give him the sports almanac, zoom in on the rear bumper of the car and the area just above it. In the time from when the front tires leave the street for the driveway to when the back tires get onto the driveway, there are several frames where you can see the blurred reflection of the camera crew on the lower rear of the car. You may think that the chrome bumper would give the best reflection, but it is actually the black area just above the bumper that gives the clearest image. (01:07:50)
Continuity mistake: During the dance hall scene, the first half of the guitar solo Marty plays (before the Eddie Van Halen section) is completely different to the one he plays in the original film. Whether this is just to give time for the jump sequence to take place or not is open to question, but all the same, although not many people would notice, it is a pretty obvious mistake. (01:26:00)