If an individual shorts stock at an investment bank, there is a limit to the amount of money the individual can lose.
The investment bank has some type of collateral, usually the rest of your portfolio. When your net worth hits a certain threshold, perhaps $0 net worth of your portfolio, the bet is over. You lost all your money, and you don't end up in debt. In practice, the investment bank will cut you off before you reach this point.
But if you are an investment bank, there is no one controlling you on a daily basis, so you can dip into the negative in theory.
The problems with doing this are:
- Regulators. Once the regulator finds out you're negative, they will shut down your investment bank. Hopefully, they do this before you hit 0, but they may not get it exactly right.
- Counterparties. Other investment banks will refuse to do business with you when they know you are negative, or close to it. Once this happens, it's game over.
So in both cases, individual or investment bank, there is a practical limit on how much you can lose from one short bet. That limit is equal to your net worth.
You could lose any amount of money up to that on one short. It's risky.
There are also ways to limit your losses. You can purchase other contracts to limit your losses. This is kind of like insurance. In general, you are not required to buy this. Some people and institutions do not do this because of the expense. Most institutions structure each deal so there is no way one deal will risk the entire business. Smart people also take steps to make sure one trade does not wipe them out.
But sometimes mistakes happen.