Timmy being right about something when the wish is still in effect.
While said wish is in effect, when Timmy guesses what is behind Francis' back he guesses something that is dangerous to have on someone. Francis assumes that is wrong, but the wish changes that so Timmy is right. Francis is then injured by the object that was behind his back.
On the map in Mr. Crocker's classroom, Wisconsin is abbreviated WD, but in real life, it's abbreviated WI.
When Chet Ubetcha is about to reveal people's secrets, the caption says "Jack" instead of "Chet".
When Timmy claims that Vicky is too busy cooking his comic books to listen to him say he has Fairy Godparents, she says he's wrong, even though the wish is still in effect meaning he should have been correct. It's possible, though, that the wish had no effect since Timmy didn't finish his sentence, as Vicky interrupted him.
Timmy outright states that there was no way that AJ would listen to him. This should have come true, due to the wish, however AJ attempts to record what Timmy says in spite of that.
There are several problems with the US Map:
Oregon is square shaped
Iowa and Arkansas almost have the same shape
North Carolina and Maryland are not on the Map
There is only one Great Lake
New Mexico Has 6 Sides and Not 8
Delaware is bigger than Massachusetts
Vermont is bigger than New Hampshire
New York City is cut off of New York and is in New Jersey