Trading on margin

[Shanemex]

You can't actually go broke on margin, can you? I thought margin calls were in place to prevent that from happening. So even if I didn't anticipate a huge drop and get off of margin, if not out of stocks altogether, then still the worst I could lose would be like 50%, right? I don't really understand margin requirements though, so I don't know at what level you need to reach before there is a call.