I made a huge mistake once.
Guy walks into a shop i had, and purchased 300 euro worth of goods. His card was a foreign card. First one didn't go throuhg so he gave me another. It wasn't a chip and pin, so while he was signing the card i was packing up his things. It was so busy in the shop i just said, thanks very much handed back the card and his receipt and went to to the next customer.
2 weeks later i got a letter from the card terminal people, saying this person was saying he never purchased anything from my shop. To prove he did i would have to send back the slip that he signed. Duh me in all the confusion with the first card not going through and the line of people behind him after he had signed he didn't actually give it back to me and i didn't even notice!!!
I had no way of proving that he was in my shop and the money got returned to his card. I was so mad....but nothing i could do about it. I heard a while later i wasn't the only person to get stung with this and they target at busy times and use a card first of all that they know wont go through just to waste time. And it's always a foreign card!!!
So always remember to get the signed receipt no matter what. This was also tried on me a few months later but i was a bit wiser, before i handed over the goods, i saw him put the receipt in his pocket he denied this saying he had given it back to me. So i refused to give him the things he had purchased, he wasn't happy especially since then he said he wanted to return everything he had purchased.... oopsss everything was on sale no refund or exchange allowed. He gave me back the receipt and walked out in a huff lol. I'm surprised he didn't just walk out with the receipt cause they could of done the same thing just this time i would of still had the stock.