It's just before 11pm when Pat Kenny is handed an explosive new piece of information. He turns to Sean Gallagher and informs him that "Sinn Féin say they are going to produce the man who gave you the cheque for five grand".
at reads out the tweet, which had been posted about 20 minutes earlier from an account called @mcGuinness4Pres, that stated: "The man that Gallagher took the cheque from will be at a press conference tomorrow."
Cue car-crash television as the frontrunner presidential candidate implodes live on Frontline.
As soon as Gallagher gabbled the "envelope" word, a loud "Oh my God!" escaped from me. It's such a loaded word for a politician to utter.
To me that was the standout moment -- not the tweet itself, but the candidate's response to it.
Over four months later, that brief tweet has come back to haunt the national broadcaster. But could the vastly experienced Kenny have done anything differently on the night?
Not really, is the simple answer.
The pressure on any moderator of such a debate is immense -- especially one with seven candidates, each of whom is anxious to get their own points across and to score points against their rivals.
Pat had his hands full, particularly when the electrifying attack on Sean Gallagher was launched by a visibly gloating Martin McGuinness.
So when he was given something to read out by his team right in the middle of the heat of battle, he could hardly have paused proceedings to check that the information had been verified.
He had to take it on faith that it was correct.
But this is the trouble with Twitter. It's a relatively new medium, and the traditional media are still grappling to figure out its place in the scheme of reportage.
It has the advantage of being an absolutely instant way of breaking news, but it has the disadvantage of frequently being the repository for information which is inaccurate, unverified or incorrect.
Twitter has become part of current affairs in media; tweets run in ticker-tape style across TV screens during panel talk shows and candidate debates; they're read out on radio and printed in newspapers.
But, unlike letters to the editor, or emails, phone calls or texts to broadcasters, the provenance of tweets isn't subjected to the checks or scrutiny that are applied to the other forms of communication.
Tweetgate was a cock-up waiting to happen. Moreover, Frontline was under pressure to deliver a dramatic debate -- they had talked up the significance of this last of a series of televised clashes, promising a "game-changer".
So how delighted the large gathering of Frontline staff must have been when it all kicked off between Martin McGuinness and Sean Gallagher.
They had on their hands the very game-changer that they had promoted.
Nobody could have predicted in the triumphant aftermath how their sweet victory would turn sour as the row over Tweetgate intensified.
It certainly wasn't seen as hugely significant upstairs in the Green Room during the debate. Most journalists from other media organisations who were reporting on the final showdown were monitoring on their laptops the rapid torrent of tweets commenting on the programme.
The majority of the posts were opinions -- from complimentary to scathing -- on the performances of the respective candidates.
This is perhaps why the tweet from @mcGuinness4Pres stood out when it popped up just after 10.30pm -- it wasn't comment, but a single- line statement that ensured that this breaking story would pick up steam the following day.
However, within about 10 minutes, Newstalk's political correspondent Páraic Gallagher had strolled out of the adjoining room and over to where I was sitting and remarked that the tweet had not come from the official account which bore the similar name of @Martin4Prez2011.
Páraic, who had checked the name against his own personal list of official presidential campaign accounts, was simply warning his colleagues that the bona fides of this tweet were dubious.
At that point, the tweet wasn't the story -- not by a long shot. The allegations thrown at Sean Gallagher, and his stumbling reaction to them -- that was the startling story of the night.
After the show had ended and the candidates had departed from Montrose, the Frontline team trickled into the Green Room for a wind-down drink and a post-mortem.
The mood was celebratory and spirits were high. Even the normally unexcitable Pat was clearly still running on adrenaline after such an intense two hours of live televised drama, as colleagues gathered around to congratulate him on keeping what had been a hugely fractious confrontation from spinning out of control.
But nobody in that room could have foreseen the storm brewing on the horizon.
And it took some time to break -- the following day as the Gallagher camp frantically tried to regroup, the candidate didn't mention the incorrect tweet when interviewed by Pat on the radio in the morning or on the News at One or the Six-One News that evening.
But as the extent of the damage to his campaign became clear towards the end of the week -- and most starkly in the collapse of his vote when the ballot boxes were opened five days later -- the tweet took on far greater significance.
However, one of the many questions raised by this controversy is: was it the tweet that did for Sean Gallagher's presidential hopes, or was it his evasive, unconvincing response to the Sinn Fein candidate's attack compounded the following day by his hostile reaction to Frontline audience member Glenna Lynch, who had questioned his business acumen?
Had Sean shot himself in the foot in the first half of the debate, before the tweet was read out live on air?
Tweetgate is a finger-pointing mess, a bitter coda to a bitterly fought presidential election. There are questions which may never be answered about who knew what, and when.
But one thing's for sure -- Pat Kenny and RTé henceforth are likely to treat Twitter like the Wild West saloon-bar that it is, where folk tend to shoot first, and ask questions later.