Factual error: When Ben Stiller gets done "cleaning the pipes", he looks around the bathroom for "the missing hair gel", it is shown hanging from his ear when he answers the door. This is somewhat unrealistic. This glob of "hair gel" would never have hung onto his ear for as long as it did. It would have dripped onto his shoulder long before he reached the door.
Continuity mistake: When the dog is on fire, Pat gets a vase of water and pours it on the dog. In a wide shot the dog is on it's back, with all fours pointed up in the air. Yet in the closeup of the water being poured on the dog, the dog is on it's stomach just sitting down properly. This is obviously due to the switch of the fake dog to the real dog. (00:46:30)
Deliberate mistake: When Ben Stiller gets done "cleaning the pipes", he looks around the bathroom for "the missing hair gel", he should have known where it was simply by his senses. Earlobes have feeling in them. He should have felt something wet on his earlobe. He also would have felt it dangling from his earlobe.
Continuity mistake: At the end of the film, when the camera is focused on all of Mary's suitors in her apartment, you clearly hear Matt Dillon say "Dom?!" off camera (as if he entered the room and was surprised to see Woogie there). A few seconds later, he bursts through the door, sees Woogie and says "Dom?!". (01:45:10)
Continuity mistake: As Ben drives up to Mary's house to take her to the prom, you see the front of the house and the front door with the door knob on the left side of the door. When the door opens though the handle would have had to be on the right. No way the door could open the way shown in the movie if the handle was on the left.