Revealing mistake: When Jim's Dad walks in on him and the girl having 'goodbye sex', Jim scrambles around trying to cover himself. At one point, as he's going from the floor to the bed, you can see the string bikini or whatever he's wearing so that he doesn't accidentally expose himself.
Deliberate mistake: During the "breaking up" scene with Jim and Michelle when they're inside the cabin, her hair is crimpy, but when she walks outside and gets into her car, her hair is straight. This is because Alyson Hannigan filmed this particular scene during a break in the filming of a "Buffy the Vampire Slayer" episode.
Continuity mistake: When Jim is sitting with his Dad in the waiting room, after he glued himself, he is holding the videotape at an angle which is toward him. The woman in the wheelchair could not possibly see the title of the video but, when they show her noticing it, the tape is being held at an angle facing towards her.
Continuity mistake: In the scene where the Asian guy is taking a piss of the balcony at Stifler's party, you see Stifler and his girl stand up when the pee hits the pillow. Then the camera is switched to the balcony where the Asian guy and a girl are talking. Then again we see Stifler and his girlfriend, but now the pillow is dry again.
Other mistake: Stifler, Finch, Oz and Jim all have radios. Stifler says during the lesbian scene that they are getting naked. This should have been heard on Jim's radio (he is under the bed), but it is not. We never see Jim turn his radio off as his hands are in shot.