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.
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.
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.
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.
Continuity mistake: In the scene when "There's shit everywhere" in the bathroom, it is taking place at night. But in the shots of Jessica's dad in the bathroom doorway, there is sunlight coming through the window across the hall.
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.
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 Harry is drinking his Slushy fast, some drips on his neck and leaves a stain. Then, when Jessica comes, the stain is gone.
Continuity mistake: When the teacher asks her "special needs class" if they have any ideas for a float, the camera switches between people and conversations and every single time this happens the Chinese exchange student changes position (resting on her elbow, arms out stretched, hands together etc...)
Continuity mistake: When the special needs class is debating a float, look at the chalkboard in the background. The word 'float' is underlined, then the next time you see it, it isn't underlined. This occurs a number of times throughout the scene.
Other mistake: If Harry was born in 1969 and is 18 when he attends school (based on "8 years later" then "10 years later"), then the year should be 1987, not 1986 as the movie states.
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.
Factual error: Throughout the movie, Lloyd and Harry often drink Slushees. However, Slushee was not a company until 1996.