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Pending orders and the free margin they quietly eat

A pending order does not use margin until it fills. On most platforms. Some brokers reserve margin for pending orders, and some do it only for certain types.

A trader with a stack of limit orders near the market might have the margin check fail when they all begin to fill together. The first few fill. The rest get rejected for insufficient margin, producing a half-built position.

In a fast move, this can leave a basket in an unintended state. Three of five scale-in orders filled, the stop is set for the planned size, and the actual exposure is different.

The behaviour is documented in each broker's rules, though rarely highlighted. A test on a small account with the margin close to the limit shows what happens.

A safer approach is to keep a generous buffer of free margin, and to know whether the broker checks margin at placement or at fill.

How does the broker treat margin on pending orders, and has a half-filled basket ever happened?

Brokers handle margin for pending orders in different ways. On most MT4 and MT5 accounts, pending orders do not use margin until they are triggered and become positions. That means a trader can place several pending orders that together would exceed the available margin if they all filled. When one triggers, it takes margin, and if free margin is then insufficient, later triggers may be rejected or the account may come close to a margin call. Some brokers do reserve margin for pending orders, which reduces free margin immediately. Checking how your broker treats them, by placing a pending order on a demo account and watching free margin, answers the question quickly.