IT was unfathomable! Or at least, statistically implausible. But before our eyes, the banks began crumpling, the FTSE was bungee-jumping and investment bankers were dumping their gear into boxes after Merrill, AIG, Lehman, and HBOS fell like dominos.

My heart goes out to those at the bottom of the Lehman chain who have watched their jobs, stock and compensation vanish just like the company. If CEO Dick Fuld thought his scary glare was menacing, he should look into the faces of his ex-employees. He double-crossed London, serving New York to Barclays’ Bob Diamond on a silver platter. Lehmanites came to Canary Wharf last week wearing “F*** you” ties, blasting REM’s It’s the End of the World As We Know It on the trading floor.

Clearly, someone messed it up big time. But don’t blame Dick, blame the duck, or something like it.

Metaphorically speaking, a swan quacked the City to death last week. The term “black swan” comes from the concept that all swans are white and is a metaphor for something that’s earth-shattering and unexpected. It could be an assassination, stock market collapse, a sunny day in London, a Tube journey without a signal failure. The point is, we have no idea what the hell is going to happen.

Except, of course, you’ll rarely see a black swan, yet in the world of finance, black swans seem to pop up almost as commonly as white ones. The sub-prime market crash is considered a black swan event that threw every economic prediction made in the last few years out the window. But in my short lifetime, there have been several “implausible” stock market swans: in 1987, 1989, 1991, 1994, 1998, 2000, 2001, 2007, 2008. Apparently, the unexpected happens all the time. I realized a few things after last weekend:

1. Finance – unlike football, the pub quiz, or picking up chicks – is not a game.

2. Bell curves are not fortune tellers.

3. Common sense is not optional. What happens when you loan billions to fools who can’t pay it back? When you lie about your balance sheet? When your bank’s portfolio is so wildly Byzantine and esoteric you can’t even value the ghastly, half-eaten bag of Gummi Bears on your desk?

4. Expect the unexpected. If life hands you a Lehman, make Lehman-ade.

After last week’s bloodbath, we are hungry for a perpetrator. But it was not that ugly duckling Dick Fuld. It is not even us City chickens, running around now with our heads cut off. It was a swan. She was black. And as sure as greed rules the City, she’ll be back.