Watch out for the double swipe.

Old fashioned carbon paper slips - they could put two in the machine and get you to sign through both. Not easy but a known wheeze. Then came machines and the double swipe was re-invented. Machines are programmed to make it hard but where there is money.......

I spotted one a couple of years back, I was struggling with my eyes and the wise guy on the till gave me my ticket and I signed it and as I walked away I noticed an extra line (petrol, crisps and what?).
When I went back to the lad he actually was stupid enough to show me the coke can (I never never drink the stuff) he had passed in front of the scanner (by accident) as I squinted at the crisp packets. I left him with the idea he wouldn't be accidenting on me again.