Well, when you owe your broker $5,000, it's your problem. When dozens of Robinhooders owe their broker $100,000, it's Robinhood's problem. In this case, Robin

Well, when you owe your broker $5,000, it's your problem. When dozens of Robinhooders owe their broker $100,000, it's Robinhood's problem. In this case, Robinhood generally applies margin calls near the end of the trading day. However, in this case, the trader's account equity went from $50,000 to -$100,000 in the span of seconds. Robinhood did in fact liquidate his position at the end of day preventing further losses, but they are already on the hook for a 6-digit sum. (Yes, legally margin debt is the trader's problem. But I doubt they will/can collect on this.)