Revealing mistake: When Mrs Arness and her dog, William Shakespeare, are at Peter's house, the dog walks over to Georgey. You can see the actor who plays Georgey had to hold dog treats in both of his hands to keep the dog near him in the scene. Confirmed in the audio commentary.
Factual error: In Steve Martin's office, when Steve is showing Eugene Levy the woman he thinks he has a date with, Steve is using an Apple Powerbook on his office desk. It cuts to a shot of the two of them looking at the screen, and then a second shot from across the desk. The Powerbook is still on the desk with the top open, but it is not turned on. When a Powerbook is powered on, the white apple on the top cover glows (even in a well lit room). The apple is not glowing.