This unknown liability on the part of the insuring parties could in turn set off a chain reaction of defaults by the parties if the combination of their credit default swap liabilities and their other liabilities exceed their assets and they are unable to get credit to meet their obligations because the credit markets are frozen. And if those defaulting counter parties' liabilities are in turn insured by credit default swaps, the next wave is going to get bigger. Lehman's default could lead to the default of parties that insured the Lehman debt, and the default of those parties could lead to the default of additional parties who insured the debts of the Lehman parties. This is one of the reasons the government is bailing out AIG: the concern that the failure of one party in the credit default swap market would lead to a wave of defaults .
It doesn't help that you don't have to own the underlying bond to buy a credit default swap for it. This fact results in the possibility that the liabilities of the Lehman credit default swap counter parties could be greater than the actual debt that Lehman issued if more than one party bought a credit default swap on the same bond.
What makes this whole thing post modern is that no one knows the extent of the Lehman credit default swap party liability because there is no central clearinghouse for credit default swaps that can provide this information. Essentially the credit default market as a whole behaved irrationally in that it acted as if the possibility of a large scale default was non-existent, and thus the necessary market mechanisms to cope with a large scale default never emerged. This was a direct result of a lack of regulation of the derivative markets.