Andy McSmith: The danger was lurking right under his nose

Characters in Armando Iannucci's farce The Thick of It live in constant fear that the mass media will catch them out – a fear that so dominates their lives that it induces irrational behaviour and causes them to bring about the very disaster they are striving to avoid.

"That was a disaster," was Gordon Brown's first reaction as he climbed into the car after he had had a few words with Gillian Duffy. Actually, it was not. That exchange in a Rochdale street was played and replayed on the news channels yesterday. The verdict is that it went quite well. But by thinking it was a disaster, Mr Brown converted it into a catastrophe.

High-ranking politicians loath unexpected encounters with voters who hold strong opinions. Labour's manifesto launch in the 2001 campaign was ruined when Tony Blair was caught outside a Birmingham hospital by a furious Sharron Storer, whose partner was being treated for cancer.

Something similar happened to Gordon Brown earlier this month when he was accosted by Ben Butterworth, who wanted to know why his children could not get into the state school of his choice.

But Mr Brown's exchange with Mrs Duffy had gone rather well, if only he had realised it. She expressed herself forcefully, he replied courteously, and it ended on a friendly note. But because Mr Brown felt uneasy while this was happening, his immediate reaction was that it had gone very wrong.

"You should never have put me with that woman. Whose idea was that?" was his next remark, which was equally revealing.

Actually, it was an unplanned encounter, but Mr Brown is so used to the careful choreography of election events that he assumed someone on his team had vetted Mrs Duffy before putting her in his path.

The person standing nearest to Mr Brown during the exchange was his long-serving chief of staff, Sue Nye, who has been at his side, watching for pitfalls, since 1992.

Finding it hard to believe that a trusted aide would expose him to such a risk, Mr Brown uttered a few succinct words to explain why he thought Mrs Duffy should not have been allowed to approach him. When Brown called Mrs Duffy "a sort of bigoted woman", it was not to imply that she was so racist that she would stand out at a BNP rally; he meant that she was bigoted when compared with the sort of voter he expected Ms Nye to line up for him.

As he obsessed over an imaginary slip-up, Mr Brown, and the adviser sitting at his side, Justin Forsyth, overlooked the more lethal disaster clipped to the Prime Minister's lapel. They had forgotten that, to avoid the crush of boom microphones that would otherwise have surrounded him, Mr Brown had agreed to be miked up by Sky News, whose technicians heard the whole conversation.

Tony Blair's battle-hardened minders would have avoided that mistake. They used to supply a Labour Party microphone, so that they could switch it on and off. He was only ever caught out once by a microphone, when he was talking to George Bush, who hailed him with "Yo, Blair".

One mistake followed another. Gordon Brown's next engagement was an interview for BBC Radio 2, with Jeremy Vine. Although it was for radio, these interviews are routinely filmed, but Mr Brown forgot that he was on camera, and inadvertently supplied the most graphic visual image of the day, of the Prime Minister clutching his head in despair.

Anyone who has been near the centre of a high-pressure election knows how easily disasters such as yesterday's can happen. For Gordon Brown and his minders the pressure is relentless. Almost the only brief moments when they can relax are as they travel from one engagement to the next, sink into the upholstery of a chauffeur-driven car and give vent to private thoughts.

It's unfortunate for Gordon Brown that this is what will be remembered when everything else that he has said in the campaign has been forgotten.