A stop price triggers an order. It does not guarantee the price where that order fills. In a fast-moving market, a $100 stop can trigger a market sell and fill at $94 if available buyers at $100 have disappeared.
At 9:30 a.m., the position still looks manageable. The chart has been moving quickly, but the last trade is near $102. Your stop sits at $100, placed before the open to define the loss you are willing to take.
Then a sharp move hits. Price trades through $100. The stop activates. Your broker sends a market order to sell.
By the time that order reaches available buyers, the bids near $100 may already be gone. Perhaps the next meaningful bid is $98. Then $96. Your fill arrives at $94.
The stop did its job: it got you out once the condition was met. It did not promise a $100 exit. FINRA explains the same distinction: when a stop order triggers, it becomes a market order, and fast markets can produce executions far from the stop price.
Educational content, not financial advice.
A documented morning when available prices vanished
On May 6, 2010, U.S. markets experienced what became known as the Flash Crash. During a period of severe market stress, major indexes fell sharply and then recovered. The joint CFTC-SEC report on the event documented how rapid selling, automated activity, and thin liquidity interacted across markets.
The important detail for a trader is not the dramatic chart. It is what happens when the price you expected is no longer available when your order arrives.
In the report’s account, Waddell & Reed executed a large sell program in E-mini S&P 500 futures. The selling interacted with high-frequency trading and fragmented market structure. Liquidity that looked present could vanish quickly, while trades printed at prices far from recent levels. The outcome was uncertain while the decline unfolded because market participants could not assume that normal buying interest would remain in place.
A stop order faces a smaller version of that same problem. Your trigger price is a condition. Your fill price depends on the market that exists after the condition is met.
That distinction matters most when you are sizing a position. If your plan assumes a $2 loss per share between entry and stop, but a fast move creates a $6 exit gap, the actual loss can be three times the planned amount.
The stop price belongs in the plan, the fill belongs to the market
A stop-market order usually prioritizes exit. Once triggered, it seeks the best available execution. That can be useful when your first priority is no longer holding the position.
A stop-limit order handles the tradeoff differently. It can set a limit on the lowest acceptable sale price, but it may remain unfilled if the market moves through that limit. You may avoid a $94 fill, while still holding an asset trading below your limit.
Neither order type removes risk. They distribute it differently:
- A stop-market order accepts execution uncertainty in exchange for a stronger chance of exiting.
- A stop-limit order controls the acceptable price, while accepting the risk that no exit occurs.
- A smaller position reduces the damage from either outcome.
The decision should happen before the position is open, when the chart is quiet and the loss feels theoretical. Waiting until price is moving through your stop turns an order-type choice into a pressure decision.
Size for the gap, not only the chart level
Position sizing often treats the stop as a fixed loss boundary. That works only when the exit fills close to the planned level.
Build a gap allowance into the risk calculation. If a trade enters at $104 with a $100 stop, the visible risk is $4 per share. If the instrument can move quickly, test the position against a worse fill, such as $98 or $96. The number is an illustration, not a forecast. Its purpose is to show whether the position still fits your maximum loss if liquidity thins.
This is especially important around scheduled news, market opens, low-liquidity periods, and assets prone to abrupt moves. A stop should be part of the risk plan. It should not be the entire plan.
For a related example of how position size can make a planned loss exceed a fixed risk limit, see [Mateo’s oversized breakout trade](\/blog\/position-sizing-mateo-s-oversized-breakout-trade-made-the-risk-visible-5093f7de\/).
Put the execution assumption in your trading journal
Record more than entry, stop, and exit. Add the expected loss at the stop, the worst plausible fill you used for sizing, and the order type chosen. After the trade, compare the assumption with the execution.
That journal entry makes slippage visible. It also prevents a common mistake: treating a stop that filled away from its trigger as proof that the stop “failed.” The stop may have worked exactly as designed. The risk model failed if it assumed every trigger would fill at the displayed price.
The May 2010 Flash Crash showed how quickly displayed liquidity can change under stress. Your trade does not need a market-wide event to face the same basic condition. A thin order book and a fast move are enough. Plan for the fill the market may give you, then size the position so that result remains survivable.
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