You'll love this one from last year.
Young woman comes into shop and after browsing for a while buys a small designer bag for 1.99 and pays with £20 note. When she has left,young girl on till says to me "I think i did the change wrong and only gave her £17.01" I'm thinking, this is not good, we don't rip people off, take a pound out of the till and see if you can find her and give her the pound. After a few minutes she comes back, can't see her anywhere. Next minute, one of the other traders comes in and says "watch out for a girl passing dud twenties" Panic starts to set in and i look at the note in the till. This is a good one, feels about right, looks about right, but something says it's not real. So off we go trying to find her, along with 3 other traders who look as worried as me. As luck would have it, one of us found her up the hill and detained her. I rushed up there, gave her my Clint Eastwood "make my day" look and managed to get my change out of her. Just at that minute a police car is driving past, which we flagged down. She was arrested and eventually did time for it. She had managed to pass off 6 of those notes in our row of shops, which goes to show how easy it was and how good the quality was.