You cannot keep issuing denials that you were informed by officials of something only to then turn around and admit you were told about that something after all.
The lapse brings back memories of Jenny Shipley's election-year clanger as Prime Minister when she had to apologise for making up a claim that TVNZ had paid $1 million to former news reader John Hawkesby as an exit settlement.
The blunder had a negative effect on Shipley's image, but National was already facing defeat regardless.
Key's misdemeanour also invites comparisons with Helen Clark's speeding police motorcade to get her to the rugby on time or the incident in which she was pinged for signing a painting and passing it off as her own work for a charity auction.
But neither event made a blind bit of difference to her or Labour's ratings.
Key's bumbling behaviour is of more significance, as it involves highly sensitive matters of state.
The saving grace for him is that, like Clark, he is seen as someone who is usually straight with voters. He has long shared Clark's approach of 'fessing up to problems and moving on.
Indeed, his biggest frustration during this affair is the time it took for the GCSB to give him the information he needed to bring matters to a close.
Such was the agency's reluctance to hurry things that Key hauled in officials this week for a dressing down.
The net effect may well be that Key will now exercise the full oversight of the GCSB that Opposition politicians have accused him of failing to provide.
Much will hang on the overhaul of procedures likely to flow from the review of the agency to be conducted by the Cabinet Secretary, Rebecca Kitteridge.
Key's recovery may be helped by him graciously accepting the word of others present that there was mention of the GCSB assisting the police during a briefing he was given on the agency's wider role and responsibilities, even though he could not recall that mention.
Some prime ministers would have stubbornly clung on to their position of denial. That would have been a serious error of judgment.
On top of that, there is the question of how much Kim Dotcom and the workings of the GCSB really matter to the average New Zealander.
The same measure applies to Key's handling of John Banks. Neither issue alone is a vote-changer.
What may be damaging to Key, however, is that his poor handling of the Banks episode has been rapidly followed by his equally woeful, if not worse performance in the GCSB-Dotcom imbroglio.
In that, the Prime Minister has been punished by the fatal combination of his indifferent oversight of the GCSB and the organisation's lax attitude to following the letter of the law.
Quite plainly, the GCSB was an accident waiting to happen. Moreover, it had clearly been an accident waiting to happen for some time.
That is apparent from the agency's audit of other examples dating back to 2009 of where it had helped law enforcement agencies.
The audit, carried out on Key's orders after his bombshell announcement that the agency's monitoring of Dotcom had been illegal, threw up three other cases out of a total of 58 where the GCSB could not assure the Prime Minister that its legal position was "totally clear".
Three cases might not sound like a lot.
In terms of abuse of power, however, one case is one too many.
The results of the audit, released on Wednesday, were inevitably overshadowed by the revelation on the same day that Key had been told about the GCSB's involvement in the case when he had claimed he had not.
The Prime Minister's biggest error was a seeming desire not to be informed of operational matters being undertaken by the GCSB and presumably the Security Intelligence Services as well.
On the day he revealed the GCSB had acted illegally, Key refused to accept ministerial responsibility for what had happened on the grounds he had been kept in the dark by the agency.
That stance was astonishing. It illustrated how keen Key was to put plenty of distance between him and the GCSB even though doing so was constitutionally untenable.
As Labour pointed out, the law that established the GCSB as a department specifies that the Prime Minister is "in charge" of the organisation, rather than its chief executive, which would normally be the case.
So the buck stops on Key's desk, and Key's desk alone.
His cavalier view of ministerial responsibility has been in sharp contrast to Clark's.
She likewise may not have been comfortable with the security services. For her, however, that was doubly reason to maintain rigorous oversight and a hands-on grasp of detail.
Key is very uncomfortable with such an approach. This week he lectured journalists on the perils and pitfalls of the prime minister getting too close to the operational decisions of the intelligence services, going as far as raising the spectre of Sir Robert Muldoon.
Being briefed on operational matters does not necessarily mean you are interfering in those operations.
But ultimately only the prime minister can stop a spy agency going feral.
That is the lesson from the Dotcom disaster. But it is not clear whether Key has learned that lesson in its entirety.
Debate on this article is now closed.</strong>
