Why it matters
Adverse selection is the killer of passive strategies. You collect small wins when nothing happens, then get filled right before the move that hurts.
Operational use: this is how the market taxes urgency. When it appears, slow down or size down.
Common trap
Judging limit orders by their fee savings instead of their markout. ‘Maker’ can still be toxic if you are consistently filled at the worst moments.
Example
You place a limit buy and get filled instantly. Seconds later price drops hard. The fill wasn’t luck; it was information: someone sold into you for a reason.
Operational cue: if you can’t point to it on the chart, in the DOM, or in your broker logs, treat it as a story and trade smaller.
Notes
Quick rule
- If you see this repeatedly, you are in a different regime than your model assumes.
- Regime change usually means position size change.
- When in doubt: trade smaller, or don’t trade.