Brokers track a single number: margin level, calculated as equity divided by used margin, times 100. A common structure is a margin call at 100% and an automatic stop-out at 50%, though the exact thresholds vary by broker and instrument.

• Position size too large for the account, so a normal move consumes the buffer.
• No stop-loss, letting a loser run until the broker intervenes.
• Several correlated positions moving against you at once — three long crypto trades are effectively one trade.
• Overnight swap and financing charges quietly eroding equity on a held position.
• A weekend gap opening past where your stop sat.

The instinct is to deposit more money. That keeps the position alive but does nothing about the reason it went wrong. Closing or halving the losing position restores the margin level immediately and, more importantly, caps the damage at a number you chose rather than one the broker chose.

Margin calls are almost always a position-sizing failure rather than a market surprise. If a single trade risks 1–2% of the account with a defined stop, the margin level never gets close to the threshold — the stop fires long before the broker does.