A limit order at the bid is not a guaranteed fill

[Amanhasnoname]

Price touching a limit level and a limit order getting filled are two different events. Plenty of scalpers learn that during a quiet drift, when the chart shows price tagging the level and the order sits there untouched.

The reason is queue priority and the bid versus ask distinction. A buy limit sits on the bid side. Charts commonly plot the bid, so a candle low touching the level only proves the bid reached it, and the fill depends on whether sellers actually hit that bid in size. On retail platforms with aggregated liquidity, the order might never even reach the real queue.

Sell limits get hit differently, because a buy order at the ask is what triggers the fill, and the ask is the bid plus spread. So a sell limit placed at the high on a bid chart needs the market to trade the spread higher.

The practical lesson is to place limit orders a fraction inside the level when a fill matters more than the last tick, and to accept that missed fills are part of the cost of waiting.

Does anyone track how often a "touched" limit level fails to fill on their broker?