A margin call is what a broker issues when your account equity falls below the maintenance-margin requirement on your open positions. You either add cash, close positions to free up margin, or the broker does it for you on its own schedule — usually at the worst possible price.
The math is brutal. Maintenance margin requirements typically range from 25% on long stock to higher on short stock and concentrated positions. A leveraged long equity book that drops 15% can easily trip a maintenance call, especially if the underlying drop happens on heavy volume with the bid-ask spread widening at the same moment.
The catastrophic feature is the timing. Margin calls almost always trigger during fast adverse moves — exactly when you'd want the option to hold positions and wait for a bounce. Forced liquidation at the bottom of the move locks in the maximum loss; the recovery that would have saved you happens to someone else's portfolio.
The structural defense is staying well clear of maintenance margin in the first place. Most pro traders run with significant buffer (50%+ free equity above maintenance) precisely because the variance of the book in fast markets is unpredictable, and the cost of getting margin-called once dwarfs the foregone P&L from running with less leverage. The accounts that blow up are almost never the conservatively-sized ones; they're the ones that ran near maintenance on the way up and discovered what 'near maintenance' meant on the way down.