Revealing mistake: In the scene where Harry and Lloyd first meet by running into each other, when it shows it in slow motion, you can see Lloyd brace for an impact when he's not suppose to know Harry is coming.
Revealing mistake: When we see the slow motion replays of Harry and Lloyd colliding, you can see that Lloyd's tooth doesn't touch Harry's head. In fact, their heads completely miss each other.
Revealing mistake: In the scene where the guys get the flagpole wedgie, we see Lloyd's coat being pulled up by invisible wires.
Continuity mistake: In the scene were Lloyd finds Harry's "treasure chest", you can see that the chest is laying flat against the wall, but when the Principal and the lunch lady come and find it missing, there is a small dirt outline, as if the chest was pointing out from the wall, instead of larger outline, as if it were flat against the wall.
Revealing mistake: In the scene where Harry is going to Jessica's house on the waxer, if you watch carefully most likely you'll catch the last person who falls is wearing roller-blades like the kind that look like shoes.
Factual error: In the scene where Lloyd puts on a record on over the loud speaker to get ready for school in the morning, it plays Ice Ice Baby. This song was first released in 1990, whereas the film's set in the 80's.
Continuity mistake: When Lloyd is at Harry's window trying to wake up with him, the shards of glass on the windowsill keep changing positions. In the final shot of the scene, they have disappeared altogether.
Continuity mistake: When Jessica is chasing the bus to the museum, her hubcap comes off after she goes airborne over the hill. It is back on in the next shot.
Visible crew/equipment: In the scene where Harry and Lloyd are drinking slushies Lloyd starts to tell Harry "you gotta go fast Harry", right then you can see something black covering up Lloyd's "cracked tooth" proving that it's not cracked.
Continuity mistake: When Lloyd is sticking the napkins on himself in the cafeteria, you can see he is wearing underwear under the napkins, while in the next shot he isn't.
Continuity mistake: When Harry and Lloyd first collide, if you look carefully you can see that Lloyd still has his entire tooth when he falls. It was supposed to be chipped.
Continuity mistake: When Ms. Heller is talking to the tour guide at the museum, she offers her a $5 bill to take the tour twice. How the bill is folded and held changes. When Ms. Heller says do the tour twice the pinched between her fingers, then it's folded in half. It keeps switching between folds. (00:35:19)
Factual error: Throughout the movie, Lloyd and Harry often drink Slushees. However, Slushee was not a company until 1996.
Continuity mistake: When Jessica and the principal are in his car watch the driver's side mirror. When Lloyd and Harry are following them you see a side view of the car and it has a regular teardrop mirror. After Lloyd and Harry get ice cream the principal is telling Jessica no one will be looking for her and the side view mirror has gone to a big square truck mirror like welded to the door.
Revealing mistake: In the shot when Harry and Lloyd are holding on the principal's car while riding in a shopping cart, the speedometer on the principal's car reads zero, while they are rolling forward with some speed.
Continuity mistake: When the bus is driving to the museum the camera goes to Lloyd and Harry. In the window you see a car passing. When the movie shows an overhead shot of the road, there is no car in front or to the side of the bus.