On the episode The Same Game, Bender looks at Timmy's teeth and he only has one tooth split in half. But later when he is looking at his teeth in a spoon, he has a mouthful of teeth! How'd he get so many teeth so fast?
If you look closely in any episode whenever Timmy smiles or talks you can see all or most of his teeth. This is because he has big buck teeth which show when his mouth is closed, when he smiles or talks his back teeth show too.