By Andrew Marr
BBC political editor
It seems to be a simple mistake by a Treasury official but it casts sharp new light on how the government is handling new demands to open up its files.
Norman Lamont was chancellor during Black Wednesday
BBC News' Freedom of Information unit had asked for details of how sensitive requests on the back of the new Freedom of Information Act are being dealt with by different departments.
Well, the Treasury sent back rather more details than anyone here had expected about its worst moment in recent times - Black Wednesday, September 16, 1992, when the pound fell out of the European Exchange Rate Mechanism (ERM), despite desperate attempts to shore it up.
Vast sums of money were squandered, the Tories lost their reputation for economic competence and John Major's government lost its heart.
In many ways, new Labour was born from the ruins.
This wasn't, in short, any old story, and the present government has already been accused of using Black Wednesday to revive painful memories for the Tories.
The Financial Times had been trying to get the documents, written by some of the most senior mandarins in Whitehall, which give the Treasury's own views about all this.
Pretty soon it will get them, but significant parts have been cut out.
The information sent to the BBC explains what is being held back and why.
Some of it involves 13-year-old private conversations with foreign officials.
One sentence, revealing that Britain got wind of a French interest rate rise, is to be excluded because the Treasury implies that Britain has been spying.
"It's not clear what the information source for this was - the source could be covert and still in use," it reads.
Other sentences have been subject of intense legal debate between officials and legal advisers.
There are references to the government's "hype about economic miracles," to the divisions inside the Cabinet.
It says "the open warfare between the Chancellor and the Prime Minister made it especially difficult for the markets to decide what the effective objectives of the government were."
There are political errors of all kinds, including Norman Lamont's relationship with the president of the Bundesbank, or "the chancellor's undiplomatic handling of Schlesinger".
And economic mistakes too - "the stamp duty holiday that had been intended to bring forward a recovery in the housing market ended up further undermining the confidence that's essential to recovery".
Sharp comments like this may not amaze students of the events around Black Wednesday, though they will displease Tory ministers of the time.
More sensitive today is the enthusiasm for withholding ancient economic forecasts vital to the full story of Black Wednesday.
This seems very odd - even the Treasury believe the Information Commissioner may disagree, but why would they be kept secret?
Answer, it would set a precedent.
Specifically, the Treasury says, it would "read across to Oliver Letwin's request", that's for material about the current chancellor Gordon Brown's forecasts for growth, which the shadow chancellor, Mr Letwin, thinks may have been "deliberately exaggerated or simply mistaken."
And to requests for internal assessments about Mr Brown's Golden Rule.
So there's sensitivity in the Treasury, and that in itself probably helps put paid to conspiracy theorists who may suspect these documents were deliberately leaked to the BBC, designed to remind us of a difficult moment for the Conservatives.
Anything is possible, but it hardly seems likely that shadowy Labour figures would release material now which points out the dangers of open warfare between a chancellor and prime minister.
No, it seems like a genuine - but revealing - slip.
Show your working, say exam papers in maths - in this case it's the very last thing they meant to show.