Usually it makes the office party go with a swing when an ex-employee who has gone on to achieve celebrity status suddenly turns up.
Peter Mandelson is a controversial figure in the wider world, but he is the most famous personality to have worked at Labour Party headquarters since Denis Healey, and can usually expect a welcome back there. A few weeks ago he was the well-received speaker at a farewell party for a long-serving member of staff.
But when he put in an appearance at a similar function last week for two long-serving employees, Nick Sigler and Richard Taylor, Mr Mandelson must have felt a frosty wind of social embarrassment pursuing him around the room.
It was perhaps fortunate that Gordon Brown appeared to have left the party before his old antagonist arrived - although some sources say he was still there - since the day had been dominated by Mandelson's description of the Chancellor as a man "obsessed with politics" who had "outmanoeuvred" Tony Blair over the euro.
But the Prime Minister was still there, with Cherie Blair and a senior adviser, Sally Morgan, another former party employee. According to one of the guests, the Blair party behaved as if the former court favourite was not in the room.
"They weren't deliberately turning their backs, but there was no conversation between them and they were avoiding being in the same part of the room. Mandelson went after about 10 minutes."
Since his second enforced departure from the Cabinet, Mr Mandelson has remained on good terms with the Prime Minister and his circle - at least until last week. Mr Blair is known to have rung him from time to time, and his assiduous championing of the European cause has been helpful to Downing Street.
But his indiscreet remarks to journalists over lunch last week left him with few friends, apart from the journalists who enjoyed being handed a good story and the management of the Quod restaurant, near London's Piccadilly, who enjoyed a bonanza of free advertising. "We always respect our guests' privacy, but this was something we had no control over," said the restaurant's marketing manager, Nadia Deluca.
Some observers were astonished that a man whose reputation was built on his adroit handling of the media should think that he could make such explosive remarks to 20 journalists without comment. Those close to Gordon Brown suspect that he did it on purpose, in the hope that laying bare the fractures in the Blair-Brown relationship would finally provoke the Prime Minster into sacking his Chancellor. What it actually provoked was fury against Mr Mandelson, much of it from people who worked with him in the general elections which made his name.
Jean Corston, a former party employee who now chairs the Parliamentary Labour Party, used its weekly meeting to call for a "period of silence" from Mr Mandelson, while his old mentor Neil Kinnock scathingly dismissed the idea that Mr Mandelson's comments were evidence of government in-fighting.
What must have come as the worst shock was the swingeing attack by two of the younger Labour MPs, Tom Watson and Jon Cruddas, former colleagues at Labour Party headquarters who are on opposite sides in the euro debate. They buried their differences to put out a statement accusing Mr Mandelson of having spent weeks "scurrying around Westminster mounting a highly personal, shrill and divisive campaign using the single currency to divide the Government, split the Labour Party and undermine the Prime Minister and Chancellor".
Both men had taken to heart a lesson which has been drilled into Labour apparatchiks over and over again for years - that if leading members of the party indulge in feuding and name-calling, support for the party could collapse, as it did in the 1970s. Last week, the former director of communications appeared to have forgotten what he once preached.