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. (00:00:30 - 00:01:15)
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: 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. (01:29:00)