One of the main factors which contributed to the Stock Market Crash in 1929, when the very loose regulations related to margin orders.
In financial terms, margin in an instrument which consists on depositing a collateral with a counterparty (generally the broker) to cover some of the credit risk that the depositor places to that counterparty.
In the 1920s, the mandatory requirements regarding margins were not very strict, and brokers asked investors to put in a small fraction of their own money. Leverage rates which measure the proportion of debt, reached 90% with a high frequency. Nowadays, the Federal Reserve has established the limit of 50%.
Back in 1929, when the stock market started to contract, many investors received margin calls. They had to hand in more money to their brokers, because the amounts required before were not enough and if not, their shares would be sold. Many people did not have the extra margin amounts required, their shares were sold and the market declined further. This generated more margin calls and more declines. This is why margin calls were one of the causes which triggered the Stock Market Crisis and, in turn, the Great Depression in 1929.
