When Jerry is showing Derek how to use the memory eraser during his job as the Tooth Fairy, when they show a shot from behind his whole arm is there, but when they show a shot from the front part of his arm is missing.
During the Tooth Fairy's first assignment, he makes his escape on the boy's skateboard. As we see him heading down the hallway, he is as tall as a low table, but seconds later, he is clearly still only six inches tall.
The lesson to learn in this story was for Thompson to believe in the Tooth Fairy. This is why he was forced to be one for two weeks. However, at the end of the film, he has his memory wiped of this experience. This would most likely mean he didn't learn a lesson by believing, letting him go back to how he felt in the first place, with a two week gap in his memory. But: when Derek is finished with his tooth fairy duties, Tracy tells him that he won't remember anything that happened in Fairyland. He learned the lesson during a hockey game, not in Fairyland. Also he remembered to go back to Randy's talent show meaning that his memory is not completely erased.