Happened to me a lot at school, only once since I left and that was from a customer I served during retail. I got annoyed and told him it wasnt cool and he actually apologised. He didn't realise I had a stutter, he just thought I was back tracking and being nervous. Still, not a very nice thing to do, but he didn't like the idea of him taking the piss out of someone with a speech impediment. Certainly didn't expect the reaction.