Read up on the hunt brothers and the silver markets. Essentially, they would buy up contracts with brokers to buy 1lb of silver on july 1st for $10 (or whatever the specifics happen to be). Then, in June they bought all the silver, like some ridiculously huge fraction of all the silver in the world. When it comes time for that broker to find that pound of silver he owes the hunt brothers, the hunt brothers get to set the price, because they're the only ones with silver. So this broker, he has to come up with how ever much the Hunt brothers demand, he is really over a barrel.
Just one memorable example of market manipulation and unlimited exposure.
Just one memorable example of market manipulation and unlimited exposure.