I've been trading ETFs for several years and would like to trade the ES, but have a question about liability. I know that the entire equity in my account could be lost if there is an extreme market move, or trading is interrupted for any reason (systems down, terrorist attack, etc). But are there clear rules regarding what the liability could be if the losses exceeds the value of your account? Let's say, just as an example, that I am trading 10 contracts with equity of $100K. If the market were to drop by 25%, like in 1987, and it was impossible for IB to liquidate my account in a timely manner because of system failure , then I am looking at a loss in excess of $150K. Can IB then hold me responsible for the additional $50K?