When Kevin and Gary are cooking burgers at the end of the movie, they flip half cooked patties, put cheese on them, and as the camera is panning away, they are flipping them again. Nobody flips patties once the cheese is on them.
When Kevin takes his hamster out of its cage while running away from the mobsters at the airport, the cage suddenly disappears.
The mobsters' Lincoln Town Car changes at one point during an aerial shot in the movie. During the shot, you see that the rear end belongs to a 1981-1984 model Town Car. While during the rest of the movie you can see that it belongs to a 1987-1988 model.
Kevin tucks a carton of eggs under his arm, goes to the fridge and gets a soda. In the next shot, the carton has vanished from under his arm.
When the bad guys catch up to the real Derek, one of them punches him in the face. As he falls his cap falls off twice.
That brand of baseball cards (Topps, 1994 edition) did not contain gum in the packs.
Kevin's pet is an Abyssinian guinea pig, not a hamster, and he is keeping it in a Habitrail (which is also too small for most hamsters).
When Kevin/Derek and Dr. Timmerman examine Mr. Pike's dental x-ray, you can clearly see that it is that of a female patient as it has two earrings visible in it.
While on the golf course, the golf cart makes a screeching noise as if it was stopping on tarmac, whereas in fact one stopping on grass would be silent.
When Derek is with the Youngs at the wine tasting, the audio says, "Me, xylophone extraordinaire." While watching his lips you can see that he is clearly mouthing something else.
When Kevin is chased by the dog, he is shouting, "Come on!" as he runs and the dog is barking, but their mouths don't move.