Bumper bonuses mean bumper tax
It's probably a serious forecasting error that means the public finances will be in marginally better shape than feared (which makes a change, you might say).
Just a few weeks after the Treasury predicted that the revenue from its one-off bonus super-tax would be £550m, bankers are lining up to tell me the government will receive many times that amount.
In fact, it is highly likely that just two international banks, Goldman Sachs and JP Morgan, will each pay more than £550m.
The numbers work like this.
A 50% tax is being levied on bonuses greater than £25,000, which is payable by the banks as a "payroll tax".
Goldman and JP Morgan are expected to declare bonuses for UK-based employees worth $2bn in aggregate each (and roughly $10bn respectively for executives across the world).
On the reasonable assumption that most of that $2bn consists of bonuses greater than £25,000, Morgan and Goldman would each pay around $1bn in tax, or more than £600m.
Other international banks, such as Bank of America, Credit Suisse, Morgan Stanley and Deutsche, would be expected to pay about 25% less.
So the yield from just those banks would be around £3bn.
And then on top of that, there should be whopping contributions from the genuinely British banks, Barclays and Royal Bank of Scotland.
Just like Goldman and JP Morgan, the tax take from each of Barclays and RBS could be more than the Treasury's £550m estimate for the entire banking industry (see my note of a month ago, RBS board to quit if Chancellor vetoes £1.5bn in bonuses).
Now, I am sure that when it comes to the expensive moment of truth, the banks will find ways to reduce what they pay.
But even if they slash this year's bonuses and also take aggressive steps to avoid the levy, the Treasury should be able to garner several billion pounds in total.
Which would be useful to it, though (not that you need reminding) nowhere near enough to fill the black hole in the public finances.
As it happens, a Treasury official, Edward Troup, said shortly before Christmas - in evidence to MPs on the Treasury Select Committee - that he expected the gross take from the tax would be around £1bn.
He said that the £550m figure announced by the Chancellor was a "net" figure: the Treasury expected the new tax would persuade some banks to reduce bonus payments, which would mean that mainstream income tax and national insurance would be reduced.
But even the £1bn looks wrong by a wide margin.
So how and why did the Treasury make this error?
Cynics in the City think it was a deliberate piece of political opportunism. They say ministers want to be able to announce some "good news" in the budget, and a bumper harvest from unpopular bankers would be classified as such.
Although some say that a more plausible explanation is that the authorities still have worryingly little knowledge of the vast sums generated and earned in the City.
I should also point out that the anger about the tax among bankers is still rising - which they won't forget (they say) next time their organisations have decisions to make about where to base a new office or new business.
The tax may be their just desert for their role in helping to cause the global financial crisis. That's certainly how millions of citizens see the impost. But no turkey ever helped to turn the oven on, come Christmas morning.
If you take a US bank like JP Morgan, for example, I am told that more than 90% of its big profit generators in London are not Brits (that's to ignore Cazenove, its venerable British stockbroker).
These proudly cosmopolitan types simply don't understand why they are being clobbered by the British taxman, and won't easily forget or forgive.
That said, most of the big banks are likely to protect their British-based employees from the full burden of the tax.
The banks' bosses don't see why staff who happen to be located in London should be penalised for what is perceived to be the bad luck of sitting at a computer screen in the one country that has announced a special bonus tax.
So the likes of JP Morgan, Goldman and Deutsche are likely to treat the tax as a cost for their firms as a whole, not just their UK operations.
The effect of that would be to reduce the bonus pool available for all employees.
Some like Goldman may also pass perhaps half of the cost of the tax to shareholders, in the form of lower dividends.
In other words, this 50% British tax on bonuses will in practice lead to a reduction of between 5% and 12% in bonuses paid to all bankers, wherever they happen to be living.
Which means that the fame or infamy of Alistair Darling and his bonus tax is spreading to bankers in every nook and cranny of the global market place.