Pushy parents who want to know why Little Johnny hasn't gone solo after 3 hours not believing Little Johnny really, really isn't interested in learning to fly.
Yuppies who think that to get a licence you just have to hand over a wad of cash.
When your star student announces his wife is expecting thier first baby and you know that they will only be doing one more lesson then gone for good.
That actual teaching is the easy bit!