OK, before chip and PIN when I went into a shop and used my credit or debit card I would have to sign a piece of paper to show that I've authorised that payment. I've always been safe in the knowledge that if the cashier accepted the transaction and I decided not to sign I could always dispute it with my bank and the money would be refunded because there would be no signature.
Now, I went into my supermarket to buy some goods. When I put my card in the slot all that was displayed on screen was "Enter PIN and press ENTER", there was no total amount. So I put my PIN in, a few seconds later the till draw opens and the cashier takes about £30 of cash out the till, at first I thought she must have misheard me when I said I wanted no cashback. She didn't give me the money, she just put it to one side.
So, I checked my reciept but there was no cashback listed so she must have took the money out for some other reason. But it got me thinking, what if she had rung £30 cashback through the till and put the money in her own pocket after I left. I would have had no way of knowing what she'd done until after I'd approved it with my PIN and after that it might be too late. If I didn't check my reciept until I got home it would almost certainly be too late.
I don't know about other till systems but the one I've used expects you to approve a transaction without telling you the amount, for all I know I could be giving them permission to take £10,000 out my account. This hasn't happened to me but if it did, what chance would I have of getting their money back?