This is pretty embarrassing, but I have a phobia of cashiers. I don't know why or when it came about, but it's a problem.
I don't think there was a particular incident in my childhood that caused this. I have no memory of a cashier terrorizing me when I tried to buy candy or something. But even at the age of fifteen I still get the feeling of someone's hand twisting around in my stomach when I need to pay for something (that's if I don't get someone to do it for me).
I get this weird thing where I think that I'm going to randomly turn into a four year old and forget how to count money and the cashier is going burst into laughter and broadcast my stupidity on the intercom. So I do this thing where I always pay with twenties so they can just give me the change and I can hightail out of the store.
So I guess the moral of the story here is that I need to get a bank card. Or start doing all my shopping online.