Revealing mistake: During the prom scene, especially during the song "(Don't You) Forget About Me", none of the band guys (especially the older man keyboard player) are actually playing their instruments.
Continuity mistake: In the bedroom scene the girl is holding a clear cup full of beer. The camera goes off her and when it comes back she is holding a blue cup. The camera goes back off her then on her and the cup is clear again.
Continuity mistake: When Stifler's mom seduces Finch, you can clearly see Finch playing with the balls on the pool table. The camera cuts to another shot, and when it's back to Finch, the balls are back in the original place.
Continuity mistake: When Finch is moping through the house, depressed, he goes through a door marked "keep out". The door handle is on the left side, meaning that from inside the room the handle must be on the right. However, when he plays with the pool balls, we can see that the door he must just have come through (there's no other door visible) has the handle on the left.
Audio problem: In the prom scene just before Heather is looking around and looking bored, the drummer is tapping on the hi hat, but the sound is a crash cymbal.
Visible crew/equipment: When Jim walks to Nadia the first time - right after he trips on the couch - a light stand is seen right at the left edge of the screen. This was pointed out in the DVD.
Continuity mistake: Around the middle of the film, Oz's shirt changes going from the car park to the library, and then back again when he's singing in the classroom.
Character mistake: When Jim gets home and finds the apple pie on the counter with a note from his mother, the note reads "you're favorite", rather than "your".
Visible crew/equipment: When Finch is running to the toilet, a microphone wire is visible trailing down the inside of his shirt. This is pointed out in the DVD commentary.
Continuity mistake: When Nadia is going through Jim's drawer in his bedroom, a tub of Vaseline magically disappears from one shot to the next. In the same scene on top of the drawer a small white box appears at the same time the Vaseline disappears.
Audio problem: When Finch rolls the yellow 1-ball around the table, the first sound you hear is that of a ball hitting another ball, yet no other balls move. No balls are pocketed either - if a ball had been pocketed the yellow ball would have taken a different path around the table.
Continuity mistake: When Nadia goes to Jim's house, Jim throws all the porn stuff messily into a drawer quickly. A little later, when Nadia opens the drawer, all the stuff has been neatly positioned.
Continuity mistake: In the famous Nadia on the internet segment, there is a quick shot of the computer before Jim starts to get undressed. If you look on the desk next to the keyboard there is a gray t-shirt. It wasn't there when Jim was setting up the camera and Jim hasn't begun to undress yet. Obviously shot after Jim threw the shirt on the Web cam and it slid off.
Continuity mistake: Finch runs into the girl's bathroom when the laxatives affect him. He tries to take out a toilet seat cover, but there are none left. The side shot shows a corner of the box hanging down below the dispenser, whereas when the shot changes to the front, there is no visible corner. It then returns to the side and the visible corner, and back to the front with no corner.
Audio problem: When the father gives the son the magazines, right before he's about to leave, he taps his hands together, but a loud CLAP is heard, like he really clapped his hands hard, while he just touched them together.