A good memory is needed once we have lied,” observed Pierre Corneille, the 17th-century French tragedian. He was right. The complexities involved in keeping an untruth plausible and consistent are so tortuous that to be really good at lying demands exceptional recall of what was said when and where.

But Corneille was writing before the age of YouTube. Nowadays, no amount of familiarity with memory's labyrinth will save you when there is downloadable disproof at the click of a mouse button. So Hillary Clinton discovered this week, when she was caught out in a prize fib about a trip she made to Bosnia when she was First Lady 12 years ago.

Dilating on her extensive experience of foreign affairs, the New York senator told a campaign event last week that she vividly remembered how, with the Balkans still a cauldron of war, she had flown into an airfield under sniper fire. She had had to dash, head-down from the aircraft, she told the spellbound audience, to the safety of waiting cars, and the planned traditional arrival ceremony had been hastily cancelled in the mêlée.

It sounded thrilling - like something out of a Tom Clancy novel. The problem was, it probably did come out of a Tom Clancy novel. It was pure fiction.

CBS unearthed some news video of the arrival ceremony and it was promptly disseminated on YouTube. There was Mrs Clinton, serene and smiling, strolling with her entourage from the plane, head held high, and in no evident danger from snipers, terrorists, or even the odd slightly miffed Serb. Seconds later she was being greeted in what looked very much like a traditional arrival ceremony on the tarmac where a small girl embraced her and the two chatted warmly for a while. I've been in more physical danger coming out of the car park at Heathrow.

Confronted with the incontrovertible evidence Mrs Clinton acknowledged this week that she “misspoke”. Misspeak is an Orwellian term deployed by politicians to describe what has happened when they have been caught in a barefaced lie.

The Clintons have a well-formed habit of misspeaking. Bill Clinton, of course, was always doing it. But his wife has also over the years mastered the art of misspeaking in what Mark Twain once described as an “experienced, industrious, ambitious and often quite picturesque” way.

She has misspoken on any number of occasions when the straight truth might have been very damaging: over her involvement in the various scandals of the early Clinton years. But alongside these instrumental whoppers, there have been some befuddlingly pointless little tiddlers too.

For no obvious reason she once claimed her parents named her after Sir Edmund Hillary, even though she was born more than five years before the mountaineer's ascent of Everest, when he was known by almost no one outside New Zealand.

When she ran for New York senator she claimed to have been a lifelong fan of the New York Yankees even though no one could recall her ever having expressed the slightest interest in or knowledge of the baseball team.

In fact the facility with which the Clintons misspeak is so pronounced that it is quite possible they have genuinely forgotten how to tell the plain truth. There was no real need for Mrs Clinton to make the claim about landing in sniper fire. But the compulsion to embroider, to dissemble and to dissimulate is now so entrenched in the synapses of the Clinton brain that it came to her as naturally as the truth would to a slow-witted innocent.

Someone once noted that the thing about the Clintons is that they will choose a big lie when a small lie will do, and choose a small lie when the truth will do. Most of the time they get away with it. But occasionally, an inconvenient truth, like a blue dress with DNA on it, or some forgotten news footage, shows up and damns them.

The Bosnia misspeak, unnecessary as it was, revealed much, however. It helped to expose a much bigger untruth Mrs Clinton has been peddling throughout the Democratic primary campaign - that her time in the White House means she has the necessary foreign policy experience to be president.

First Ladies don't acquire real foreign policy experience. We know that Mrs Clinton did not, as she claimed, play a large role in the Northern Ireland peace process, that she was not, as she claimed, a key voice in counsels on the Balkans, and that she did not even have security clearance in the White House for the most sensitive of conversations about national security.

So the problem with the ripping yarn about the Bosnia snipers is that it offers hard evidentiary disproof of improbable claims about her role during the White House years.

With this latest deceit stripped away, there is not much left to Mrs Clinton's disintegrating campaign for the Democratic nomination. It capped a bad week for her, a week that might have signalled the end of her hopes.

With a deft speech that was somewhat lacking in complete honesty itself, Barack Obama last week seemed to have acquitted himself quite well, for now, of the charge of being an associate of a ranting, anti-American black preacher. More important, the collapse last week of efforts to schedule a new vote in Florida and Michigan, two states whose earlier primary votes have been disqualified, was deadly to Mrs Clinton. It is now virtually impossible for her to finish ahead of Mr Obama in the delegate count when the primary season ends in early June.

That really ought to be that. After that final primary in Puerto Rico on June 1, Mr Obama will have won more states, more delegates and more popular votes than Mrs Clinton. How in those circumstances can Mrs Clinton claim a moral case for staying in the race?

Her answer is to persuade the party's super-delegates - top party leaders and elected officials who will have the casting votes - that she is more electable than Mr Obama, and that they would be doing the party a favour if they chose her over the wishes of the tens of millions of people who have voted in the primaries.

They are unlikely to be taken in. They are more likely to view it as another example of Senator Clinton's misspeaking.

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