Slippage
Slippage is the gap between the price you ask for and the price your order is actually filled at. It can be negative or positive.
Slippage happens when price moves between sending the order and its execution, or when there isn't enough liquidity at your level. It mainly hits market orders and stop orders, which become market orders once triggered.
It is common during releases (NFP, CPI, central bank decisions), at the Sunday evening open and in thin markets. A stop placed at 1.0800 can therefore be filled at 1.0792.
An honest broker passes on positive slippage too. A limit order never slips against you: it is filled at your price or better, but it may not be filled at all.