when in a hole stop digging.
After Boris Johnson admitted he did not know the price of a pint of milk, David Cameron should have spotted the warning signs.
Asked what the price of a value-sliced white loaf was, the Prime Minister suggested it was "well north of a pound", revealing: "I don't buy the value sliced loaf; I've got a breadmaker at home, which I delight in using."
But when radio presenter Nick Ferrari pointed out the price of a value-sliced loaf was, in fact, 47p, Mr Cameron got his political shovel out: "Look, I'm trying to get my children to eat the sort of granary and they take it, actually, they like my homemade bread.
"You set the timer overnight so when you wake up there is this wonderful smell wafting through your kitchen. It takes 30 seconds to put in the ingredients."
The backlash was predictable.
Labour vice-chair Michael Dugher said: "David Cameron is so out of touch he's almost a parody of himself...
"The only surprise is when he said 'I have a bread-maker', most people will have assumed he had his own personal baker rather than a machine," he added.