As Gordon Brown learnt to his cost with his 'bigoted woman' remarks, a politician must always assume that the microphone is on.
David Cameron forgot this simple rule ahead of a speech in Leeds today, where he was overheard muttering: "We just thought people in Yorkshire hated everyone else, we didn't realise they hated each other so much."
It's also notable for just how much posher he is when he thinks people aren't listening. That'll also go down well with the Yorkshire folk.
He later attempted to backtrack, telling the BBC's Test Match Special, "I said to an aide that I thought Yorkshiremen had it in for other people rather than each other, clearly as a joke. I've been absolved by two of the greatest living Yorkshiremen; Geoffrey [Boycott] and Dickie Bird both agreed it was a joke."
We're not so sure that the rest of Yorkshire will look on it quite so kindly - after all, they've surely not forgiven him for his appalling attempt at a Yorkshire accent last October, at the Conservative party conference.