Like a substitute teacher finding out a class had missed out on a core lesson, the Prime Minister suggested that he would solve the situation.
He would remind Labour of its previous pronouncements on the economy.
That would show the party the error of its ways, he implied.
At least it would, er, just as soon as he could lay his hands on it.
That, perhaps, is the lesson when you want to throw something in your opponent's face.
It helps to have that something close by.
Labour's shadow chancellor Ed Balls, to whom all this was directed, need not have fretted.
There followed a rather long and uncomfortable silence, after which the Commons chamber broke into pandemonium.
Mr Cameron, it appeared, was struggling to find what he was looking for in the rather large bundle of notes Prime Ministers use to help them deal with the onslaught of questions at PMQs.
The Conservative leader started to go red in the face.
This set Labour MPs off even more.
At one point the Glasgow Labour MP John Robertson was told to calm down by the Speaker John Bercow.
The scene started to look not unlike a very loud mime performance.
Labour MPs started holding up their order papers and offering them to the Prime Minister, as if to say 'here, maybe it's in here?"
One Labour frontbencher even stood up and looked over the despatch box and down on the floor as if desperately helping the Tory leader find what his missing piece of paper.
When he did finally quote Ed Balls words to him the shadow chancellor saw on the frontbench nodding furiously.
Lesson, it is fair to say, most definitely not learnt.