Audio problem: When Stifler is talking to Finch about how he is going to get with Cadence and does the nipple milking impression, during a good portion of the shot that is shooting over Stifler's shoulder to show Finch's face when Stifler is talking, watch Stifler's lips from the side: you can see they are moving out of sync with what we hear, it was obviously changed and dubbed over.
Continuity mistake: When Stifler is in the linen closet it is completely dark. Finch and friend open the door and the closet is lit. After that they close the door, but the inside of the door remains lit. When the shot changes to the other side, the closet is completely dark again, without a source to light the inside of the door.
Audio problem: In the bachelor party scene, when Michelle's dad notices the "mud" (chocolate topping) he says, "We are going to need a mop." Then Michelle's mother says, "I saw it in this closet" except that her mouth does not match up with the audio (it looks like she says "It's in here."
Continuity mistake: When Stifler drives to the flower shop to get new roses, he sees the shop, slows the car, and comes to a stop while viewed from the side. The next shot is a frontal view of him sitting in the car green-screened, and all is good except that the car is moving again. The following shot then has a side view of the vehicle at a full stop.