But it's a safe bet that when he got back to the office he wanted to know what idiot came up with the crazy idea of asking the public what they thought of a New Labour flagship policy.
I'm told that the culprit's defence - ‘it will add to the number of people visiting the Downing Street website' - didn't go down too well with the PM. Even madder was transport secretary Douglas Alexander, who has been working flat out to develop a new road pricing policy.
Scottish-born Alexander knows better than most that it is a bad idea to ask the public what they think about paying more to drive. The last authority to do that was Edinburgh City Council. It held a referendum on a London-like congestion scheme - and 75 per cent voted ‘no'.
Someone else upset with the Downing Street petition is the chair of the Commons transport committee, Gwyneth Dunwoody. On the Today programme, she suggested that an ‘under-16-year-old employment scheme in Number 10' was to blame for the gaffe, but she was wide of the mark.
It was actually Peter Mandelson's former little helper Ben Wegg-Prosser who came up with the barmy idea.
Wegg-Prosser is also in charge of Blair's departure, which explains a lot. When the only line in your CV is that you once carried the bag of the Prince of Darkness, then perhaps working for Tony Blair is your only career option.
These days, the Prime Minister can't be too choosy about who he employs, because now he's on the way out he just can't find decent staff. All his best people, such as Alastair Campbell and Anji Hunter, have long gone and no one with half a brain wants to work for someone who will be out of a job in a few months' time.
It's little wonder then that the handling of his departure has been such a PR disaster, but there is little reason for Downing Street to panic about the Blair legacy. This week saw the start of Michael Cockerell's three-part history of Blair's premiership on BBC2, and there are similar shows to come.
The Prime Minister will certainly come out well from all these TV documentaries. And his current advisers may not have the brains to know it, but history is always a lot kinder to those who have departed. Not long to wait now.