Continuity mistake: When Carlotta brings the plates for the meal they all have a dome cover with a top handle, but when Sebastian runs across the table those handles have changed. (00:54:50)
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. (01:27:19)
Continuity mistake: At the end of the final match, the announcement is made it's the first time in history the defending champion kept his title. Yet in Karate Kid 1 Johnny Lawrence had already won the championship twice and was there for his 3rd time.
Continuity mistake: When Annie and Ray are laying in the newly completed ball field, and Annie is drinking a glass of wine, it is sunset. The sun has gone down and the sky is just the faintest of pink on the horizon, the rest of the sky is a strong blue tone. Yet, when the camera cuts to a long shot, same angle of Ray and Annie, the horizon is now a brilliant red-orange taking up nearly a third of the sky. (00:16:20)
Continuity mistake: When the reindeer gets into the house and runs around, it knocks over a TV set. All of the magazines that are on top of the set come crashing to the ground, including a TV Guide with "Get Ready For A Fabulous February" (or something like that) in big letters on the cover. Interesting since the movie is set around Christmas time.
Continuity mistake: When Amy is in the kitchen cleaning, she starts mopping the floor, then dancing with the mop held upside down. The mop starts out wet, but when she's dancing it's dry (either that or she would be getting dirty mop water in her face while the mop is being flung around).
Continuity mistake: When Wallace races to get crackers, check the packet's logo. When the camera angle changes it's different. (00:08:57)
Continuity mistake: The snowman hits his nose with a hammer, completely bending it in half. In the next shot it is suddenly magically straight again.
Continuity mistake: When Charlie and Itchy are escaping from the pound they are trying to dig under the fence. Someone is shooting at them and blows a huge hole in the fence. When the camera cuts to another shot of them going under the fence, the hole is now gone.
Continuity mistake: When Brian spends his first night in the underground monster world he goes in wearing a shirt and boxer shorts. But a later scene shows him wearing a shirt, pants, and shoes. He then goes back home and is back in his shirt and boxer shorts.
Suggested correction: It's entirely possible that he was able to borrow a pair of pants while down there.
I just watched the scene where Maurice and Brian pull the pranks to get the kids into trouble and he is in his pyjamas. Later in the monster world he is in a full outfit including a shirt, pants, and shoes. Then he goes back home and he is in his pyjamas again.
Continuity mistake: When Sam is swinging the shovel at Putnam's car he busts the driver's side headlight twice.