[stygiansonic]

Margin calls were one aspect that magnified the magnitude of the 1929 stock market crash.[1] After that, margin regulations (Regulation T) made the minimum margin rate 50% for stocks, though it appears some brokers may have been increasing margin requirements to near this level before the 1929 crash.[2]

1. http://stocks.fundamentalfinance.com/stock-market-crash-of-1...

2. https://en.wikipedia.org/wiki/Margin_call

[dnautics]

It's not just margin calls but leverage in general that turns downcycles into crashes, if you believe Kindleberger (and you should). [1]

1. http://www.amazon.com/Manias-Panics-Crashes-Financial-Invest...