Seann William Scott improvised some of his dialogue, this can be observed by his castmates' reactions in some scenes, such as when he tells Jessica he wants to know how many women she has slept with that year - Natasha Lyonne's amused reaction is genuine.
In the scene when Alyson Hannigan (Michelle) is "breaking up" with Jim, her hair is styled differently than normal. This is because she was shooting an episode of Buffy - Im Bann der Dämonen (1997) and she ran over to the set during a break.
Eddie Kaye Thomas was laughing so hard at Seann William Scott's antics during the lesbian scene that he had to be cut out of a lot of the shots. This is why these scenes contain a lot of shots of just Stifler and Jim, even though Finch is supposed to be standing right next to them.
Missing from the theatrical release is a scene showing the band camp leader with a rash around his mouth, the morning after having played the trumpet that was inserted into Jim's ass.
Adam Herz: in the scene with Kevin's brother (Casey Affleck) in the hallway as the two businessman talking to his right.
J.B. Rogers: in the scene with Kevin's brother (Casey Affleck) in the hallway as the two businessman talking to his right.