Revealing mistake: In the scene where Doc Brown is following Biff into the tunnel, as Marty starts to pass between the cars on his hoverboard, one of the wheels on the rig supporting the hovering DeLorean is visible.
Continuity mistake: After Gerry gets smacked in the face by the 'broken' suspender clasp, we see the silver clasp land under the dresser. Yet, when he's moaning and lifting his leg to get onto the bed, we can actually see the clasp still intact on the front suspender that came loose, which is dangling right behind him (just look between his legs). Of course, in the next shot, that clasp is gone. Since the silver clasp is one of the plot points, this is bizarre to say the least.
Continuity mistake: When John and Catherine are in the hangar at the runway, the Cessna's tail number is N3035C. When the plane is shown in the air, the number is N3973F. When they land, the tail number has changed back to N3035C.
Continuity mistake: In the bedroom scene the girl is holding a clear cup full of beer. The camera goes off her and when it comes back she is holding a blue cup. The camera goes back off her then on her and the cup is clear again.
Continuity mistake: When Sam is in his bedroom, after he finds the Cube sliver it falls and dramatically burns a hole through the floor into the kitchen. But Sam's bedroom is over the living room (there is nothing on the ground floor below Sam's elevated bathroom, and in the first movie Jazz hides in this open space), and the kitchen is actually located between the den and dining room, at the other end of the house. If they were to follow the actual floor plan of the house, the Cube sliver would have landed in the area of the piano in the living room, but where would the fun have been in that?
Character mistake: In the final fight at Goodison Park, the commentator says 100,000 people are in attendance. Capacity of Goodison Park is 40,000, and even allowing for people in the "pitch" area around the ring there's nowhere near space for that many, unless there was another layer of fans standing on people's shoulders.
Factual error: The main character is being wooed by Lockheed - an aircraft company, whose representative then gives him gifts for his kids - one of which is a desk model of an aircraft, however it is of an aircraft produced by one of Lockheed's competitors - Grumman, which would be akin to a GM rep giving a toy Ford Mustang as a gift. Both companies continue to remain competitors as Lockheed-Martin and Northrop-Grumman since the government vetoed a proposed merger back in 1998, and at no time have the two companies collaborated on a major project.