Financial Crisis:

Banking crises
When a bank suffers a sudden rush of withdrawals by depositors, this is called a bank run. Since banks lend out most of the cash they receive in deposits (see fractional-reserve banking), it is difficult for them to quickly pay back all deposits if these are suddenly demanded, so a run may leave the bank in bankruptcy, causing many depositors to lose their savings unless they are covered by deposit insurance. A situation in which bank runs are widespread is called a systemic banking crisis or just a banking panic. A situation without widespread bank runs, but in which banks are reluctant to lend, because they worry that they have insufficient funds available, is often called a credit crunch. In this way, the banks become an accelerator of a financial crisis.

Examples of bank runs include the run on the Bank of the United States in 1931 and the run on Northern Rock in 2007. The collapse of Bear Stearns in 2008 has also sometimes been called a bank run, even though Bear Stearns was an investment bank rather than a commercial bank. The U.S. savings and loan crisis of the 1980s led to a credit crunch which is seen as a major factor in the U.S. recession of 1990-91.

Speculative bubbles and crashes
Economists say that a financial asset (stock, for example) exhibits a bubble when its price exceeds the present value of the future income (such as interest or dividends) that would be received by owning it to maturity. If most market participants buy the asset primarily in hopes of selling it later at a higher price, instead of buying it for the income it will generate, this could be evidence that a bubble is present. If there is a bubble, there is also a risk of a crash in asset prices: market participants will go on buying only as long as they expect others to buy, and when many decide to sell the price will fall. However, it is difficult to tell in practice whether an asset's price actually equals its fundamental value, so it is hard to detect bubbles reliably. Some economists insist that bubbles never or almost never occur.

Well-known examples of bubbles (or purported bubbles) and crashes in stock prices and other asset prices include the Dutch tulip mania, the Wall Street Crash of 1929, the Japanese property bubble of the 1980s, the crash of the dot-com bubble in 2000-2001, and the now-deflating United States housing bubble.