Many people are aware that banks cannot survive a bank run - an event where too many customers decide that they want their money out please. If the bank doesn't have enough cash-in-hand to pay them then they can only close their doors, making the problem obvious to everybody. Trust in that bank implodes and then everybody wants their cash out.
The question of course is - what is it that prompts the original few to start withdrawing their money and kicking off the process? A mere rumour may be enough.
Another means by which a bank may be caught short is the "complex financial instrument" (good wording: "complex" implies that it's way beyond our simple minds to understand, so please leave it to the experts) - otherwise called a "derivative" - an agreement that in theory will hedge your risk against some unforeseen event that would otherwise cost you a great deal of money. Banks make good money selling this form of insurance whilst such events do not occur, and so with blithe insouciance create an unknowable network structure of risks and counter-risks that nobody controls - unknowable that is until some of those risks turn bad and trigger a tsunami of claims and counter-claims that the system as a whole cannot contain.
In these febrile times, the risk that that tsunami will be triggered is perceived to be rather high, so best be informed.
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