http://en.wikipedia.org/wiki/Bucket_shop_%28stock_market%29
The highly leveraged use of margins theoretically gave the speculators equally large upside potential. However, if a bucket shop held a large position on a stock, it might sell the stock on the real stock exchange, causing the price on the ticker tape to momentarily move down enough to wipe out its clients margins, and the bucket shop could take 100% of their investments.