Sun and Moon

Slippage

The gap between the price you asked for and the price that actually filled.

It happens when the market moves between sending an order and executing it, and it is worst around news and at the weekly open. It can fall in your favour, but usually does not, because the speed is in the direction of the move. This is why a calculated loss at the stop is treated as a floor, never a guaranteed ceiling.

Work out your loss at the stop