Paul Krugman tells us to go read Felix Salmon:
Financial Risks in Lehman Collapse - Portfolio.com: How big is settlement risk right now? An indicator one might look at is the number of "fails to deliver" and "fails to receive" reported by primary dealers in U.S. Treasury bonds. Last week, before Lehman was expected to file for bankruptcy, fails to deliver rose by $351 billion to $410 billion; fails to receive rose by $336 billion to $389 billion. (PDF here; Excel file here.) Again, there have been bigger spikes in the past, but they haven't happened at the same time as the bankruptcy of a primary dealer. Right now is the last time that anybody wants to be worrying about failed trades, because they can't have any assurance that their counterparty will still exist by the time they're all worked out.
Lehman Brothers has more than $600 billion in assets that will need to be liquidated as part of its bankruptcy. That's an order of magnitude greater than any bankruptcy the world has ever seen: No one has a clue how to even get started on something so huge, let alone what the repercussions will be. Is there $600 billion in cash sitting on the sidelines of the global financial markets just waiting for an opportunity to snap up assets on the cheap? No. So as Lehman's assets get liquidated, asset prices in general, and bond prices in particular, are likely to be under a great deal of pressure
In turn, that's going to hurt other players in the global financial system, from hedge funds and sovereign wealth funds to small- and medium-sized regional banks. Anybody who's leveraged and who marks their assets to market is at risk of margin calls and possible bankruptcy themselves, depending on the volatility and risk profile of those assets.
The upshot is a state of radical uncertainty: as Paul Krugman says today, "nobody knows what will happen next."... There is a very, very long list of things that could go horribly wrong from here on out. The liquidation of Lehman is one; the possible collapse of American International Group is another. Beyond that are countless hedge funds and other financial institutions which, collectively, present significant systemic risk.
But the biggest and most obvious risk of all is the one associated with Lehman's own debt, which is now trading at less than 35 cents on the dollar. That's a big loss for the institutions holding it—but it also means an unknowably huge loss for anybody who wrote credit protection on Lehman Brothers at any point over the past five years. Those sellers of credit protection are staring down the barrel of billions of dollars in claims, and they're going to have to raise that money quick by selling anything they can get their hands on—and that might well include stocks.