The real Achilles heel is that banks lend long and borrow short. In other words, they fund long-term loans with checking account deposits and 90-day loans. These funding sources can evaporate overnight, as Bear Stearns found out. This imbalance in asset/liability management has killed countless banks throughout history.
James Turk on Citibank's fight for survival.
Jim Cramer, less than a week before the Bear Stearns crash: "No, No, No Don't move your money from Bear, that is being silly, don't be silly, Bear Stearns is fine, Bear Stearns is not in trouble".