Just as the big banks put the finishing touches to what was billed to be another season of resounding results, up pops HSBC - reputedly the wiliest of them all and an excellent parent of the erstwhile Midland Bank – with a reminder that, with all their clever computerised credit-pricing systems, bankers can still make horrendous mistakes.
In the event, the stock market decided not to make a meal, of it. Shares in HSBC are a stake in a sure-footed global colossus with a dominant presence in Asia. It is likely to be boring, but that can be a virtue in a bank. American mortgages are a side-show, or so we supposed. Anyway the shares had such a poor run through 2006, so there was no froth to blow off the top.
One reason the shares languished was underlying unease about Household, an American consumer lending outfit that HSBC took over in 2003 and re-named HSBC Finance. It had been in trouble at the time and the $14.8 billion price tag was reckoned to be a bargain.
The worry was that it concentrates on "sub-prime" lending to borrowers mainstream banks turn away, many of them hispanics, not what HSBC does in most of the world. It says it has an edge with immigrants from Latin America who are familiar with the name at home.
It looks, though, as if the trouble has been as much the American housing market as HSBC's chosen customers. It is not that US house prices collapsed - as some pundits predicted - they just stopped rising after years of giddy inflation fuelled by Alan Greenspan's one per cent headline interest rate.
When the Fed cranked up the cost of borrowing again, most existing home-buyers were all right. Millions had taken the chance to re-mortgage when money was dirt cheap, often at fixed rates for up to 30 years. They are sitting pretty. That may explain why the American housing market did not collapse.
It also explains why many of HSBC's dud loans were taken out in 2005 and last year. They were secured on homes whose prices failed to go up, so there was little or no equity to support a re-financing to help with fast-rising repayments on variable-rate loans. Two strange things about this.
First, it seems to have happened very fast - since HSBC's last trading statement on December 5.
Secondly, whatever is going on must be widespread and severe.
These are mortgages, even if some are second mortgages, after all secured on real estate, which even in a bad cases should cover a fair bit of the outstanding debt.
Considering that HSBC's share of the market is not huge, that looks like a lot of US home-buyers in trouble. Yet to all other appearances the American economy is proving more resilient than seemed likely six months ago.
What, if anything, this means for the British housing market, we shall discover when our house prices finally stop rising faster than other prices and everybody's pay - towards the end of this year, perhaps.