Direct answer
Slippage limits show up when the concept is used as if it were precise or predictable. Slippage only describes what happened (or could happen) around execution; it does not guarantee a future outcome. Its size and direction depend on conditions at the moment of execution, including liquidity, trading costs, and how an order is processed.
Mechanism or definition
Slippage is the difference between the price you expect to receive and the price you actually receive when an order is executed. The “expected” price is an assumption made before execution. That assumption can come from a quoted price, a previous price, a mid-market reference, or a planned order level.
To reason about slippage, separate stable mechanics from variable conditions:
- Stable mechanics: a trade can only execute at the prices available at that instant.
- Variable conditions: the available prices change continuously with market activity, spreads, and execution frictions.
Because the “expected price” is an input, any slippage number depends on how that input was chosen and measured. Different reference points can produce different “slippage” results for the same execution.
Evidence or example
Consider a simple illustration with clearly stated assumptions. Assume an order is placed expecting an execution at 1.1000. If the executed price is 1.0996, the slippage is 0.0004 in price terms.
Now note the limitation: the expected 1.1000 must be defined. If instead you had used a different reference (for example, the bid instead of a mid price), you might compute a different slippage for the same 1.0996 execution. Also, even if historical fills around similar hours averaged out, that pattern does not prove that future fills will behave the same way. Price availability at the time of execution is what ultimately matters.
Limitations and risks
Key limitations and failure modes include:
- Assumption mismatch: using an “expected” reference that is not aligned with the execution venue or the actual quote type. That can make slippage look worse or better than it really was.
- Timing and speed sensitivity: during rapid moves, the available prices can change before execution, increasing uncertainty.
- Liquidity and spread effects: when liquidity is thin or spreads are wide, small changes in demand can move the next available execution price.
- Execution friction and costs: slippage can be confounded with trading costs and other execution effects. Two executions with similar price deviation can have different total trading impact.
- Non-predictive history: historical relationships between spreads, volatility, and slippage do not establish future results.
These issues mean slippage is best understood as a variable outcome driven by the execution moment—not as a fixed, model-ready quantity.
Verification or next question
To independently verify whether slippage occurred, compare the pre-trade reference price you used (the “expected” price) with the actual execution price recorded in your order history. If you cannot identify what reference price was used, the computed slippage may be incomplete or misleading.
A useful next question is: what exact price reference is being used when people estimate slippage (quoted price, mid price, bid/ask, last price), and how does that reference relate to the execution price you can actually observe?