I'm ashamed to admit I've done this too.
When I was a nasty little teenager working weekends at Sainsbury's, we were shown that passing a magnetic card too close to the spinning motor inside the bar code reader would wipe its data.

So, of course, it became a standard thing to do to unpleasant customers. It was very easily done, just pop their card on top of the barcode reader box while you handed them the slip of paper to sign.

It's not big or clever. Fortunately it's not really random or evil, just a bit vindictive and wankerish.
( sanityclause is waiting until, Fri 17 Feb 2012, 18:19, Reply)