Anybody who has ever put a piece of clothing in the washing machine, only to remember too late there was a £10 note in the pocket, should empathise with this story from the BBC website
According to the article over 32,000 people demanded a total of £33.9 million from the Bank of England last year, trying to reclaim cash that had been damaged in some way. Money that has been through the wash was only the third most popular avenue of destruction, beaten by notes that have been torn or ripped in first place, or chewed and eaten (by pets, it helpfully points out) in second.
Perhaps the oddest method mentioned is the money damaged by putting it away for safe-keeping. Decaying under the floorboards I can just about understand. But damaging money because it had been kept in a microwave (and presumably used without taking the money out)? If that's the case, I'm not sure you deserve your money back.