But one man has been proving them wrong for four decades.
In the summer of 1975 Barney Curley, a fearless and renowned gambler, masterminded one of the most spectacular gambles of all time with a racehorse called Yellow Sam. It cost the bookmakers millions of pounds.
They said that it could never happen again.
But in May 2010, thirty-five years after his first coup, Curley staged the ultimate multimillion pound-winning sequel.
The Sure Thing tells the complete story of how he managed to organise the biggest gamble in racing history – and how he then followed up with yet another audacious scheme in January 2014.