Slippage
Slippage is the difference between the price you expected an order to fill at and the price it actually filled at. It happens when price moves, or liquidity thins, in the instant between order and execution.
Market orders and triggered stop losses are exposed to it because they accept the best available price. Limit orders are not — they fill at your price or better, or not at all. Slippage is largest around news releases, session opens, and weekend gaps, and it can occasionally work in your favor.
It is a normal execution cost rather than a platform defect. The practical response is to plan for it: assume stops can fill a few pips worse than placed in fast conditions, and budget for it alongside the spread when judging whether a setup is worth taking.
Example
You are long one lot of EURUSD with a stop at 1.0880, planning a 20-pip, $200 loss. A news spike jumps price through the level and the stop fills at 1.0877 — three pips of slippage. The realized loss is 23 pips, or $230.
In a prop-firm challenge
Evaluation rules are judged on filled prices, not intentions. If a stop was set to lose exactly your remaining daily allowance, three pips of slippage can push the day past the daily loss limit and end the attempt. Leaving a buffer below the limit, and avoiding market orders during major releases, are standard defenses — see news trading rules at prop firms.