The banks have been "unfair" charging more than £12 when people are late with the minimum payments on their credit cards. So says the Office of Fair Trading.
It has said so with such effect that the big banks which have been imposing fees of £20 or more for these lapses have caved in one after another this week - at the end of a two-month consultation period in which it seems the OFT threatened to sue them if they didn't.
Yet what is magically "fair" about charges of up to £12 and "unfair" about anything more? The OFT's answer is commendably precise. "We feel that the £12 - or less - is a figure that a bank could take without making money on people's defaults" they told me.
The idea is that it is "fair" to charge what it costs to chase up a late payment, but "unfair" to make a profit out of it.
The banks are wary, it seems, of confronting the OFT in court on this kind of issue because it recently won a case about guarantees on faulty goods bought abroad with a credit card.
Not that they have the least intention of making their credit card operations less profitable.
They have been making £300 million a year, it is said, from punishing slack payers.
They will now recoup that money some other way - it is known as the "water-bed effect". Barclaycard says it will charge higher interest on authorised credit card borrowings repaid on time, possibly 2.5 per cent more.
There is even talk, once again, of levying fees on current accounts that never go into the red at all.
This may be a triumph of sorts for the OFT, but it is hard to see it is one that benefits consumers.
The principle that it is not a bank's business to punish forgetful customers is all very well.
Indeed, you can argue that it is the banks who should be punished for marketing their cards recklessly and failing to encourage cardholders who don't intend to borrow to pay off their balance each month by direct debit.
In practice, the effect is to make banking services more expensive for the much larger
number who open their correspondence even if it looks like a credit card statement.
The banks say they are going to be more hard-nosed with the OFT's next target, charges for overdrafts.
There are other issues here - not least that overdrafts are shockingly more costly than they used to be when the bank manger might treat you to a glass of sherry while you haggled politely over whether you should pay three per cent, or perhaps four per cent, over the Bank of England's base rate.
In those days, though, bank profits were not measured in billions.
Credit scoring has replaced the bank manager - and made overdrafts available to all sorts of people he would have turned down out of hand. Most abide meticulously to the terms laid down by the banks and wider availability of credit is a great social and economic benefit.
But greater risk comes with bigger numbers. So everybody pays more for their overdraft.
They will pay more still of the OFT bans penalties for those who slip up.