It was the most-leaked Budget in recent memory as the Liberal Democrats negotiated in public and tried to claim ownership of their favoured measures. The unexpected burst of openness should have worked to George Osborne's advantage. His allies deployed the usual trick of trailing in advance announcements that would have got less coverage if they had been held back until Budget day. Nothing new there then.

But the Chancellor made a fatal mistake. The key measures leaked out – the cut in the top tax rate and increase in the tax-free personal allowance to £9,205. But one big one did not: his freeze on the age-related allowances for pensioners.

Unwittingly, Mr Osborne turned the Treasury's usual communications strategy on its head. Most Chancellors save a nice white rabbit to pull out of their hat with a flourish. Mr Osborne's good news for 24 million people had been well-trailed and the main item of "new news" – the so-called "granny tax" – was a dead rabbit. It came as a surprise and so received much more coverage than if it had formed part of the speculation.

Yesterday, Labour forced David Gauke, the Treasury minister, to make an emergency Commons statement on leaks. He appeared to rewrite the rule on pre-Budget purdah, insisting: "I am not aware of any minister briefing any market-sensitive information." But he was right to accuse Labour of "synthetic outrage". The man leading the Opposition attack was Chris Leslie, once an aide to Gordon Brown. That really is the pot calling the kettle black.