World markets have been in crisis for weeks. That should mean rich pickings for someone. One of the oldest, but truest, aphorisms in investment is that you should "buy when there's blood in the streets".

There is speculation that the legendary investor Warren Buffett, who is sitting on a huge cash pile, is about to start spending it. And hedge funds started buying up stricken subprime lenders earlier this year, to a flurry of publicity. But the history of those deals is problematic.