[tspareme]

The issue is expected loss. No one knew what is the expected loss in a crisis. This is why some of the Lehman debt holders actually made money from the bankruptcy. This is why TARP actually made money. It is because there is a substantial difference in asset price while in crisis and not in crisis. The asset price is what is making the bank solvent.

Of course, the central bank (unless you are locked into a monetary union of course) can lend freely during a crisis. However, it must also be careful as to not trigger inflation or worse yet cause people to lose faith with your currency. There is also moral hazard as well but that's more of a soft issue.

The bigger issue is what happens if your bank liability is many times larger than your countries GDP. This was the case with Iceland or Britain. Then you can't print enough money to make your bank whole.