It had all started so promisingly with DUP leader Arlene Foster and Sinn Fein's Stormont leader Michelle O'Neill seeming at ease in each other's company during a rare appearance together at a Conservative Party fringe event.
he mood music was, if not exactly upbeat, at least not discordant. Progress appeared to have been made, albeit at a glacially slow pace, on the talks aimed at restoring devolution to Northern Ireland.
But with one ill-chosen remark Ms O'Neill changed the whole tenor of the meeting. Her acidic aside that the North is not British brought the natural response from Mrs Foster that of course it is.
The Sinn Fein leader's comment was either thoughtless by a person not regarded as a noted strategist or else deliberately provocative.
But what it showed most of all is that Sinn Fein has little feeling for what is dear to its political opponents. The clue is in the name - they are unionists, they believe in and hold dear to the union with Britain.
Politically, whether Sinn Fein likes it or not, Northern Ireland is British and will remain so until a majority of people in the province vote to say otherwise.
Just as peaceful republicanism is a legitimate political ideology, so too is unionism.
Sinn Fein argues that a lack of respect from unionists lies at the core of the current stasis at Stormont, but Ms O'Neill's remark shows a lack of respect towards unionists who form the majority political opinion in Northern Ireland.
Ms O'Neill says Sinn Fein is keen to restore a devolved administration at Stormont. How does she think that denigrating the political status of unionists furthers that aim?
The last nine months of stalemate since the Executive collapsed has been pockmarked by caustic, inflammatory exchanges between the parties. Just when it seemed that finally the politicians were knuckling down to serious discussions, along comes another unnecessary insult.
The one saving grace was that Mrs Foster did not react in kind, ensuring that further fuel was not added to this latest fire.
However, she noted that it was the sort of remark that could have been made in March, which was a low water mark in relationships between the two parties.
This is now a time for cool heads and considered comments by politicians on all sides as it is obvious nerves are raw. Yet ground has been lost in the discussions and needs to be made up rapidly.