Factual error: The chiropractor that Ben Stiller's character visits says "Tender fascial tissue left of L7" as he is treating his back. However there are only 5 Lumbar Vertebra in our back, L7 doesn't exist.
Factual error: Ben Stiller takes Cameron Diaz to the prom in 1985, but picks her up in a car made well after that; it has the newer-style headlamps. (00:09:15)
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.
Factual error: When Magda uses her police scanner to listen to Healy's conversations, she explains it just picks up cellular phones, This is incorrect - in the 90s, police scanners picked up conversations on "cordless" landline phones (in a short range), but not cell phones.
Answer: Because he is ignorant and doesn't know anything about disabled people, he actually doesn't realise what he is saying is offensive.