My mate once installed a new graphics card in his Dell for better WoW experience, but after doing so couldn't understand why the screen wasn't working. All the lads were going round his for pre-drinks later, upon arrival he told me of his dilema. I went and had a quick look. He'd installed the new GPU correctly, just failed to disconnect the monitor cable from the on board graphics and over to the new GPU.
So not exactly a customer, but still a good one.
Another one, when I used to work in Costa Coffee at uni, I was working the coffee machine and the guy on the till was getting some hassle from a couple of customers. Apparently the day before they had come in, purchased coffee's some cakes and a bag of macaroons. They drank their coffee's ate their cake, left and forgot to pick up their macaroons on the way out. So here they were, trying to tell my mate that they were taking a packet of macaroons for free because they had left them on the table. Assistant manager comes up, tells them they can't and its their fault for leaving it on the table and they bugger of out the shop.