A gang of counterfeiters get a new extremely expensive printing machine, and at great expense, buy some plates, which they were assured were 99.9% perfect.
They buy a large consignment of paper which also was virtually identical to the real paper.
Gleefully, they switch on the machine and print a few off. They were delighted with the quality of the paper, the inks, and the printing itself – it would fool the best investigators in the whole world – if it wasn’t for the fact that the notes were all £18 notes!
What were they to do? Eventually, someone suggested going over to Ireland, and changing some of their £18 notes – those stupid Irish wouldn’t realise they were being conned!
A few days later, and they’re in a small Irish pub in the back of beyond, and they ask if they could change one of their beautiful £18 notes.
“Sure, no problem,” says the barman. “Would you like 3 £6 notes or 2 £9?”