I have a student, who has paid up front for all of his PPL. He has participated in all of our groundschool but has never flown!
We still have all his money and keep asking him when he's going to fly. He gets very irate and tells us not to keep bothering him!
Truly the dumbest I've come across.
However the ones who want to pull the collective up on entering an auto or who freeze in a flare are not much better.