In Shrek 2 Pinocchio is trying to avoid lying by using double negatives. He knows where Shrek is. He says "I don't know where he's not." This is actually a lie (though his nose doesn't grow). If he knew where Shrek was he would know everywhere Shrek isn't. You can't just randomly throw negatives into a sentence and expect it to be a double negative.

Edit: It was Shrek the Third, not Shrek 2.