While calling on Gordon Brown to organise a meeting of party leaders to sort out the mess over MPs' expenses, he pretended not to have noticed whether Nick Clegg was in the House. ("Is he here?") Of course Nick Clegg was in place: it would have been extraordinary if he had not been there.
I suppose the meaning Cameron was seeking to convey was that Clegg is so insignificant as to be beneath his notice. This sort of thing may go down well in the circles that Cameron has always moved in, but it is unlikely to impress the average voter.
Interestingly, there is no trace of this remark to be found in the official record of yesterday's exchanges between Cameron and Brown. It's a reminder that Hansard often preserves what MPs would like to have said rather than of what they actually did say.
It may also be a sign that Cameron has thought better of this tactic. Discussing the odd dispute at PMQs in February over Titian's age at his death - the one a Tory researcher thought he could settle by altering the artist's Wikipedia entry - Michael White wrote:
Dave tries to keep the Flashman side of his nature under control and usually succeeds. In constantly reminding voters that Brown finds it very hard to say sorry he's making a damaging point.I think Mr Cameron needs to try a little harder.
In accusing Brown of "never getting his facts right" over Titian while not getting them indisputably right himself, Cameron scored an own goal. Voters notice these things.