Revealing mistake: When the investigator takes Ted's head and starts slamming it on the table after Ted got finished telling them about the hitchhiker, Ted's head never hits the table but his own hand. (00:59:55)
Continuity mistake: The crowd that watches the guy fight with Warren and Ted near the beginning keeps on changing. People move where they're standing between shots and the arrangment of the crowd changes throughout the scene between shots too. (00:05:15 - 00:06:10)
Continuity mistake: When the dog is dead, Pat tries to revive it by using some wires. When it fails, he puts the wires on the couch then peeks into the kitchen to see what Mary and Magda are up to. Then in the next closeup of the dog, the wires how moved to laying on his stomach. (00:46:15)
Deliberate mistake: The man kissing Cameron Diaz in that last shot (when the homeless guy shoots the drummer) is not Ben Stiller, noticeable by his much taller, slender figure. (01:52:40)
Continuity mistake: When Ted and Mary are sitting on top of the car, Mary looks at her watch to see what the time is, but if you look closely, her watch is facing the camera, so there is no way she could have seen what the time was. (01:30:35)
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)
Continuity mistake: Ben Stiller gets "hooked" with Warren's fishing line, getting a hook that quite noticeably has the prongs sticking out of his cheek when they are removing it. In the next scene, however, no scar, and no sign of the injury exists. (01:36:10)
Continuity mistake: Tucker shows his ID bending its left side in the wide angles, yet in the close-up it appears straight.
Visible crew/equipment: Just before Warren's fishing hook gets stuck in Ted's lip, the camera pans from showing Warren fishing to Ted and Mary. Here you can see the reflection of the crew on the ground and finally on Ted and Mary when they are standing there.
Continuity mistake: When Ted sees Mary for the first time in years, the hood on his sweatshirt changes from his back to his right shoulder. (01:12:10)
Continuity mistake: When Mary has lunch with three friends, in a wide shot one of her friends picks up her drink and brings it to face level. In the following closeup of her, she brings the drink to her face again. (00:28:40)
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: Ted makes fingernail marks in the floor when Puffy is pulling him along. Two things wrong: His fingernails aren't even touching the floor when he makes them, and the marks seem to disappear in almost all of the long shots. (01:32:30)
Continuity mistake: When Ted enters Healy's apartment and hides behind the door, there is a nearly full bottle of vodka on the table. It is full of caramel-colored liquid. A minute later, when Ted brandishes the bottle to threaten Healy, the bottle is empty.
Continuity mistake: Near the beginning of the film when the time period is 1985, Ted is with his friends and the camera shows Mary biking into school. After the fight Ted offers to walk Mary and Warren home but Mary doesn't have her bike with her.
Continuity mistake: When Mary and her friends are eating lunch, Pat is sitting behind Joanie. There is enough room for someone to walk through. At the end of that scene, when Mary says that she has a vibrator the distance between Pat and Joanie has increased by double if not tripled.
Visible crew/equipment: Ted confronts Tucker and Healy at Healy's apartment. After Tucker says they need a drink and Ted is yelling at Healy, a red object with a black dot appears on Healy's face for an instant.
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.
Continuity mistake: When Ted is fighting with the dog, the carpet under the table changes back and forth between flat and bunched up.
Answer: Because he is ignorant and doesn't know anything about disabled people, he actually doesn't realise what he is saying is offensive.