Trivia: Dropping Ted while he was on the stretcher was not intended, it accidentally collapsed. When it did, the Farrelly brothers called cut to make sure Ben Stiller was alright, and then just left it in.
Trivia: The man in the prison with the long hair that stretches his arm out of the bars while Ted is leaving is Cameron Diaz's father.
Trivia: When Mary is golfing and she meets Matt Dillon's character for the first time, the woman that walks by is the director's mother. (00:27:35)
Trivia: The reason Ben Stiller's character wears a Providence t-shirt in the movie (while he is driving, before he picks up the hitchhiker) is because the Farrelly brothers are from Rhode Island originally and went to Providence College.
Trivia: When Ted pulls into the rest area and the actors are not yet visible, the hitch hiker asks Ted what his car seats are made out of cause they're itchy. But if you look at the hitch hikers face as the camera pans inward toward the car, the hitch hiker's expression does not look like he just asked that question. That line was added in later because the directors thought it took too long for the car to pull in.
Trivia: Some people may have missed this reference. In the scene after Warren gets hit in the eye with the baseball, Ted shows up at Mary's house with a ball, and says "signed by Tony Conigliaro... shipped up from Boston." Conigliaro was a baseball player with the Boston Red Sox who suffered an infamous black eye after getting beaned with a pitch.
Trivia: One detective in the scene at the police station with Ben Stiller says "I'm Detective Stabler, this is Detective Krevoy." These names are a reference to the film's producers, Steven Sabler and Brad Krevoy.
Answer: Because he is ignorant and doesn't know anything about disabled people, he actually doesn't realise what he is saying is offensive.