My question would be simple: if I mess something up in online Forex really badly, can I lose more money than I deposited? (e.g. lose my house, gain a debt)
If you don't use leverage you can't lose more than you invested because you "play" with your own money. But even with leverage when you reach a certain limit (maintenance margin) you will receive a margin call from your broker to add more funds to your account. If you don't comply with this (meaning you don't add funds) the broker will liquidate some of the assets (in this case the currency) and it will restore the balance of the account to meet with his/her maintenance margin. At least, this is valid for assets like stocks and derivatives. Hope it helps!
Edit: I should mention that
- I'm not an expert
- Always be cautious and know what you are doing when investing in the stock market/FX/etc.
FX is often purchased with leverage by both retail and wholesale speculators on the assumption daily movements are typically more restrained than a number of other asset classes.
When volatility picks up unexpectedly these leveraged accounts can absolutely be wiped out. While these events are relatively rare, one happened as recently as 2016 when the Swiss National Bank unleashed the Swiss Franc from its Euro mooring.
You can read about it here: http://www.reuters.com/article/us-swiss-snb-brokers-idUSKBN0KP1EH20150116
Contrary to what other people said I believe that even without leverage you can lose more that you invest when you short a FX. Why? because the amount it can go down is alwasy limited to zero but it can, potentially, go up without limit.
See This question for a mored detailed information.