Here's one from our DVD rental section:
There was a guy registering with us. He got a customer's card (those are free) and had to choose a four digit pin number so that, if he lost the card, nobody else could rent on his account. The card actually isn't that important, we have several customers who've lost their cards, and we can find their accounts by name, but they always have to enter the pin number.
Okay, that guy did all that. Rented something, went, brought in back on time, all well and good. He rents once or twice more, no problem.
But then, one night, and it's still the same week that he registered, he came into our store completely wasted. I didn't say anything when he was browsing the shelves, because he might have gotten upset if I wouldn't rent him something on account of him being drunk (and therefore, actually, not legally competent), and he was a registered customer, and if he rented something it would cost ten Euros at most.
Finally, the guy came to the counter, wanted to rent something. He gave me his customer's card, I scanned the barcode on it to get to his account, and asked him to enter his pin. At this, he looked at me with this drunkenly confused look, and said: "But I just gave you my card."
It took me a beat, then I told him that he needed to enter his pin number, that the card was only to find his account, his pin was the only way to unlock it so that I could rent the DVDs to him.
He did not understand. "Why don't you let me rent those DVDs? I gave you my card."
I tried to explain it to him more slowly and in even simpler terms. At this point, he was in tears, and he asked me why I was mean to him. I tried to explain again, and at the same time explain that I wasn't mean to him, but that the computer wouldn't unlock his account without his pin. "So put in the pin then," he told me. I said I didn't know the pin, only he knew the pin. Then he teared up and called me mean again. After about another five minutes of this, he left crying. He's never come back to the store again.