Continuity mistake: Near the beginning, when Jason is done writing his paper he staples it. When he and Marty are in the limo, the pages are not stapled.
Continuity mistake: When Marty is done speaking to Monty while driving to the bogus address, after he pulls on the phone wire, in the shot from his car's interior we can see that the wire actually ripped, but after nearly colliding with "grandma" the phone wire is again intact. (00:44:30)Super Grover
Continuity mistake: During phase four, when Marty opens the passenger door of Jaleel's speeding car he wears a baseball cap, but in the next wideshot his head is bare as he hits the ground and rolls - the baseball cap is gone, it didn't fall off. After a close-up of Jaleel, it cuts back to Marty and the baseball cap is back on his head just before it falls off, as he continues to roll. (01:06:40)Super Grover
Continuity mistake: When Jason writes the paper and then staples it, there are neat/short bits of paper at the side edge, where the pages were torn from the notebook (also visible after he slams into Marty's limo). Then, when Marty finds the paper, the entire left side edges are filled with much longer extra bits of paper. (00:06:50 - 00:09:55)Super Grover
Other mistake: Towards the beginning of the movie, directly after Jason's parents leave the house, Jason's sister's boyfriend drives up in an SUV to pick her up (the music in his SUV is blaring). The window on the passenger's side door is open. When Jason's sister opens the passenger's side door to the SUV, the already blaring music becomes louder, as if much more sound is escaping the vehicle. When she shuts the door after entering the vehicle, the music becomes quieter again.
Continuity mistake: When Marty enters the home of the birthday party, there is a large area rug lying directly in front of the set of double French doors. In following shots, this large rug has moved closer to the front door (before all the kids jump on him) - note the table beside the French doors. (00:45:00)Super Grover
Continuity mistake: When Marty Wolf pulls up behind "the Masher", an old lady hits Wolf's car and pushes it into the Masher truck. The old lady drives off while insulting Marty. The Masher drives around to crush Marty's car. When it cuts to Marty watching the Masher drive towards him, you see the old lady's car behind his again.
Continuity mistake: When Marty is up on the roof with Jason after being busted as a liar, he looks down at Marcus Duncan standing in front of eight live feed monitors, but in Duncan's close-up there are fourteen monitors. The entire top row of monitors is gone in the wideshots. (01:17:55)Super Grover
Continuity mistake: After Marty leaves the birthday party and asks Monty to reschedule with Marcus Duncan, when his car's radio starts blaring Marty's wearing his seatbelt in the shot facing him, but in the next side shot as he tries to turn off the radio Marty's seatbelt is instantly off. (00:47:35)Super Grover