The Great Crash on Wall Street (1929) saw a long period of relative prosperity after the first war, to be followed by shares rising and banks lending money, often to their own subcontractors or individuals in order for them to buy more shares. The subsequent spiral of rising share prices could not be justified forever.  Banks realised that their returns from lending were worth less than they had originally loaned.  Quickly, individuals and banks tried to sell their shares, failing, and this triggering a long period of difficulty for money borrowing because as lender would not accept poorly valued stocks as a guarantee.  (Galbraith, 1929).  The second war, with massive Government spending halted this crash altogether.  The expansive spending made it worthwhile for bankers to lend to companies that would be producing products for the second war (e.g. guns, tanks etc) and investment in those companies through public offering of shares.  Money would come through to higher wages and low unemployment, which would spread Government spending widely in economy.  

This apprent answer to a bubble and crash became standard consideration  as Keynesian spending, which would be used to expand the economy. However, at later dates its was realised that this also expanded the economy of all other countries at the same time simply by sucking in foreign imports.  As such it was realised that some kind of balance was always necessary between massive spending any long term damage it might do.  This monetaristic approach is now also considered. 

The Dot-Com bubble (early 1990s) took place as computerization by individuals was making it cheaper for individuals to buy on-line.  The imagined damage that this would do to retail sales caused many investors to change over from retail to computerized selling.  At the time this simply was not ready for the market at all, and money in many jumped-up companies just were shown to be valueless.  But the money had been spent often by the City seeing little Return on Investment (ROI).  This was particularly clear in the UK.

The sub-prime mortgage bubble (2009) was caused by low interest rates and the possibility of lending money to potential house buyers as mortgages when a few of these buyers, if defaulting, could be covered by plenty more that had more money than needed to cover those bank losses.  The rise in interest rates saw a much larger proportion unable to pay the mortgage and throwing in their house keys to the mortgage lender.  House prices crashed and a growing number of mortgages could not be covered even by the value of the house itself.   Lehman Brothers was the biggest coverer of other lenders so itself went bust.  This was mainly in the U.S. but the UK was badly affected when building societies had followed the same path.