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What happens if stock purchased on margin plummets below what I have in the brokerage?
Different brokerages have different house rules for margin requirements and margin calls.  You will likely get a margin call giving you a small amount of time to deposit the required funds to bring your account balance up to the required margin requirements.  In reality, a stock that falls from $50 to $4 in a short period will probably become unmarginable.  In short, yes, you will owe the broker for the loss.