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When You Trade on Margin, You are at the Mercy of Your Broker

When you are trading with borrowed money, your broker has a say in how much risk you take. As such, your broker can buy or sell at its discretion, which can be a bad thing for you. Let's say you have a margin account, and your position takes a dive before rebounding to all-time highs. Well, even if you have enough cash to cover, some brokers will liquidate your position on a margin call at that low. This action on their part can cost you dearly.
Source: http://www.investopedia.com/articles/trading/04/081804.asp