Best one I ever heard was about 15 years ago when a guy parked his brand new jaguar with an underground parking valet company in London whilst he went away on business for two weeks. Came back and collected his car all polished and cleaned. Had it serviced at 6OOO miles 4 months later and the garage said the engine had over 100,000 miles on it. Turned out the garage swapped his engine out whilst he was away on business.