Factual error: Cadence arrives for the wedding on a 727. Closer examination of the aircraft's livery reveals it is a Kitty Hawk Airlines aircraft. This would be fine were Cadence air freight, as Kitty Hawk is a cargo airline.
Visible crew/equipment: Right before the wedding Michelle is talking to Jim's father about her vows. Behind them on a table there is a mirror and you can see the arm of a crew member reflected in it several times.
Audio problem: When Stifler is driving his car away after he wrecked the flowers, he says, "Fu**ers, it's not my fault," but it's visible through the window that he's not saying anything.
Continuity mistake: In the first scene of the movie, when Jim and Michelle are in the restaurant, they show Michelle with her napkin folded flat on her lap. Later when Jim tries to get Michelle to find the ring the napkin is back on the table folded in the formal restaurant triangle.
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.
Continuity mistake: During the bachelor party, the cop stripper rips open her top and shoves Michelle's mother's face into her chest. In the next shot the stripper's top is buttoned.
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 Jim's dad goes to see Michelle (when she's trying to write her vows), her hair and makeup are done for the wedding, and her hair is in a French twist. Later, when Michelle and Jim are having their first dance, her hair is in a curly bun sort of style.