When Janie's boyfriend arrives, he clearly switches off his engine and removes the keys, but the engine can still be heard. When he gets in the car again, the keys are in his hand but the engine is still running.
When Jason is talking to the people who hate Marty, and Kaylee starts talking about making him cry, she is standing next to Jason, but in the shot right before she starts talking, she is standing on the other side of the screen, next to Monty.
When Jason finishes writing his story "Big Fat Liar", he staples the papers together, but when Marty picks it up in his limo, the papers are not stapled. Also, the writing of the title looks different from when we see Jason writing it (especially the "i" in "liar").
After Jason and Kaylee have taken some time to look around at the warehouse that they use for their temporary home, Jason says that they'll hangout there until closing time and then go shopping. When he's saying this; behind him is a telephone booth. Reflected in it is a crew member in a white t-shirt reading some sort of newspaper or maybe the script to the movie. He is looking down at the pages then seemingly looking up at the actors as they say their lines. (Maybe he's the script supervisor or something.)
Jason liberally applies a super glue-type product to the earpiece of Marty's cell phone hands-free set. Any type of super glue product should dry pretty quickly after it's applied. MArty finished swimming and took a shower as well as freaked out when he saw himself. By the time he stuck the ear piece into his ear, the glue should have at least been tacky if not dry. It would have been "easily" removable.
When Wolf realizes his car has been sabotaged (specifically where he finds that his brakes are connected to the car horn), his brakes clearly work as he stops his car (despite hearing the car horn when he hits the brakes) but immediately following we see he cannot stop the car.
When Marty is upset that his car has been sabotaged and is banging away at the instruments, in the close up of his hands you can see he is wearing blue mechanical rubber gloves instead of the blue died skin.