What appeared in April last year to have been a constructive ambiguity about the relationship between decommissioning and the formation of the Northern Ireland executive eventually became a trap which caught the two key parties.
When the agreement was negotiated, the danger of this happening was not clear to most people, myself included.
True, I noted the side-letter which David Trimble had extracted from the Prime Minister, Mr Tony Blair, at the end of the negotiations. This was to prevent or limit any breakaway from the Ulster Unionist Party which might follow from the absence of any provision in the agreement for a date of commencement, as against a date of completion, of decommissioning. This letter, however, contained an obvious and potentially dangerous ambiguity in its reference to the time when decommissioning "should" begin.
This is because such a side-letter could not alter the terms of the agreement and the use of the ambiguous auxiliary verb "should" was obviously designed to sound as if it meant more than it possibly could in the circumstances.
Yet given that the provision of such an ambiguous side-letter appeared to be the only way to get the UUP on board, its provision then certainly seemed defensible.
Indeed, the mood of the moment encouraged the hope that the two sides would sort out what appeared to be a residual sequencing problem.
There were two factors which prevented most of us - and perhaps even the two governments - from understanding then just how potentially dangerous that ambiguity would turn out to be.
First, Mr Trimble had signed the agreement despite Jeffrey Donaldson's walkout, and there was also the tough line he took with dissidents next day at his party meeting.
These combined to distract attention from the dangers inherent in his stance having become so dependent upon such a flimsy piece of paper.
It was only later that it became clear that, when it might have been hoped that he would have been setting out to sell the agreement enthusiastically to unionists, what he perceived to be the weakness of his position was leading him to impale himself on the hook of decommissioning being started before the establishment of the executive.
Second, it took a long time for it to become clear that the key to the Sinn Fein leadership having been permitted by the IRA to sign an agreement was the provision which postponed the start of decommissioning until after the formation of the executive.
It now seems clear that without this provision, Sinn Fein could not have signed the agreement. To expect it to give way later on the very point on which its participation in the agreement depended was probably always unrealistic.
Perhaps seeing later how firmly the UUP had embedded itself on the prior decommissioning hook, Gerry Adams and some of the Sinn Fein fellow negotiators may have been tempted to consider such a concession, especially when it became clear that the whole peace process which they had initiated might founder on this issue.
Yet the IRA, with its different decision-making composition, was not going to allow this to happen.
Helping unionists off self-impaled hooks is just not what the IRA is all about.
What we do not know is just what was said by Sinn Fein negotiators to their IRA colleagues when decommissioning was discussed during their preparations for the negotiation of the agreement.
Indeed, was there ever a consensus among all concerned that decommissioning would happen at some stage? Within the Sinn Fein/IRA decision-making process, was this issue fudged, just as it was to be in the talks between the parties and the governments?
Even if decommissioning had been fully accepted by the IRA as something that might happen at a later stage, was that to be conditional on IRA satisfaction with the Patten Commission's ultimate proposals for RUC reform?
We simply don't know.
What we can be sure of, however, is that whatever "seismic" move the IRA may have finally agreed to make five weeks ago - involving the initiation of decommissioning immediately after the formation of the executive - must have been made with great reluctance and with many misgivings as to whether it would deliver the goods.
Yet that move, hyped by two very relieved governments, failed to evoke a positive reaction from a UUP which was then in considerable disarray.
Inevitably, those in the IRA and in Sinn Fein who had all along believed that unionists had never been serious about sharing power with Sinn Fein and the SDLP, must have emitted a very loud: "we told you so". This left Mr Adams and his fellow negotiators in a highly uncomfortable position.
LOOKING back on events we can now see that a number of factors combined to work against the successful implementation of the Good Friday agreement.
First, the two governments, which for so long had worked skilfully and unitedly together, seemed towards the end to lose their way.
Perhaps they were betrayed by desperation and exhaustion into a series of ill-judged moves.
That is not to suggest that it would have been humanly possibly for anyone else to do better: some challenges are just too great to be successfully taken on by mere humans, however dedicated.
Given UUP disarray, and the obvious doubts which must have existed about whether there were any circumstances in which Mr Trimble could at that late stage have brought his party with him, the two governments may have been unwise to have then pressed for an indirect indication by the IRA that decommissioning would begin in the immediate aftermath of any early formation of an executive.
It may have been a mistake for Mr Adams and his colleagues to have sought and secured authority for such a move from the IRA when the UUP was likely to turn down this ultimate concession.
The truth is that trust has now been damaged all round: between the SDLP and UUP; between the two governments and the two contending Northern parties; and, it would seem, within Sinn Fein/IRA.
It remains to be seen whether, in the light of the ensuing debacle, those who have led Sinn Fein and the IRA so near to a peaceful settlement will now retain the authority to pursue this matter further.
One must presume that next month George Mitchell will seek to revive the "deal" which failed to materialise five weeks ago.
Logically, he must seek to persuade Mr Trimble and enough members of his party to agree to the formation of an executive on the basis of a revived IRA commitment to initiate decommissioning immediately after that event.
Simultaneously, despite what has been said by the IRA and Sinn Fein leaders that decommissioning is now off the agenda for the foreseeable future, Mr Mitchell will need to persuade the IRA that such a belated climb-down by the UUP would represent a victory substantial enough to justify reviving the "seismic shift" of July.
Either of these acts of persuasion on its own would be an extraordinary achievement for the US mediator.
To persuade both sides simultaneously to act along these lines would seem to ask the impossible, especially, perhaps, from the UUP.
Yet such an outcome now seems the only way forward for unionists just as much as for nationalists.
Unionist political leaders have always seemed to allow their emotions - fear of absorption into what they see as an alien Irish state, and hatred and distrust of an IRA which has killed several thousand of their community and maimed countless others - to distract them from identifying objectively where their ultimate interests lie, and from pursuing these interests clear-sightedly and cold-bloodedly.
For some time this failure has been putting at risk the ultimate survival of their community.
In the absence of peace and political stability, very many of their young people go to Britain for higher education and do not return. This phenomenon does not exist on anything like the same scale among nationalists.
It is this annual disappearance of almost two-fifths of the better-educated unionist young people, rather than a mythical out-breeding of Protestants (fertility rates of the two communities are now at about the same level), which threatens the majority position of Protestant unionism in Northern Ireland.
Within weeks the UUP will be faced with what could turn out to be a final choice between recreating a future for further generations in a peaceful and prosperous Northern Ireland, or effectively pulling the plug on such a prospect.
In the worst case from the UUP point of view, should the IRA refuse to reiterate its offer of early July or fail to deliver on it, its negotiating position with the two governments, and the moral strength of its position vis a vis world opinion, would have been immensely strengthened.
In the best case, it would have secured the future of a stable, peaceful and prosperous Northern Ireland.
It is this reality facing Mr Trimble and his negotiators which gives Mr Mitchell a chance of succeeding in this final throw of the Northern Ireland dice.