Mechanism
- A market order crosses the spread and can sweep depth.
- In fast moves, the book updates while your order is in flight.
- Your broker may fill via multiple liquidity sources with different latencies.
Microstructure note: stops fail most often at the same time liquidity disappears. That is not bad luck; it is structural. Your job is to avoid competing for fills in the worst queue.
- Prefer “don’t trade” windows over cleverness: rollover, open/close, data prints.
- Reduce size before you reduce stop distance. Size is the only lever that always works.
- Measure slippage by regime, not by average.
How it kills accounts
Market order in thin/fast conditions → worse average fill → R multiple collapses → you “need” a bigger move → you hold longer → you absorb the reversal.
How it kills accounts:
- Edge looks fine in backtest.
- Live spreads widen at the exact wrong moments.
- Stops trigger inside noise, so you widen stops.
- Same size + wider stop = silent leverage increase.
- A normal spike becomes structural damage.
Rule that survives
- Default to limits unless speed is truly essential.
- When using market: pre-define max slippage and abort if exceeded (platform-dependent).
- Reduce size when the book is thin. Don’t pay for speed and size at once.
Rule that survives:
- Spread is a gate, not a footnote. If it’s abnormal, you don’t trade or you trade smaller.
- Assume worst-case fills in fast markets.
- Size is the adapter: reduce size before changing the stop model.
Example archetype
You “just need to get in” on a breakout candle. Market in, slip, then the breakout fails. Now you’re down immediately and your stop is effectively tighter than planned.
Tell: if the trade only works when the spread is tight and price is smooth, it’s not an edge, it’s a regime bet.
Deep dive
Deep dive
Market orders are fine when liquidity is thick and stable. They are dangerous when liquidity is conditional.
Think of them as buying speed by selling price certainty. That trade-off is sometimes correct, but it should be deliberate.
Glossary pointers: price improvement, requote, market impact.
Field checklist
- Measure spread before entering. If it’s abnormal, you’re trading the wrong product at the wrong time.
- If volatility expands, reduce size first. Don’t “solve” it by widening stops with the same size.
- Avoid the predictable liquidity holes: rollover, session open/close, first minutes after data.
- Assume your stop may fill worse than your entry. Price the worst-case, not the brochure.
- If you cannot explain where liquidity comes from, trade smaller.