Derren Brown did a brilliant variation of this a couple of years ago, persuading punters that he had an infallible horse betting system. All completely genuine, although laborious to film - he started with something like half a dozen race meetings and selected six-horse races with suitable odds, then got each of his punters (who never saw the others) to bet on a different horse. It carried on over several weekends and at the end of it there was, inevitably, one lass who had had been given six winning tips in a row. For this last one she had been told to scrape together every bit of spare cash she and hubby had in their current accounts ... and the look on her face when Derren Brown explained that she was simply the mathematically certain 'winner' out of over 100 losers and that if she did bet any more the chances were that she would lose the lot!