Slippage is the difference between the price you asked for and the price you got. Sometimes you get better. Sometimes you get worse. Mostly, in quiet liquid hours, you get exactly what was on the screen.
The interesting moments are not the quiet ones. News prints. Session opens. Weekend gaps. Anywhere the order book thins out or moves too fast for quotes to keep up. That is where slippage shows itself.
On a stop lossStop LossAn exit order that closes a losing position the moment a chosen price level is hit.Click the word to learn more, slippage tends to bite. By definition, price was racing against your positionPositionA single entry held on the broker account, with a direction, a size, and a current floating result.Click the word to learn more when the stop triggered, so the fill lands past the level. On a take profitTake ProfitAn exit order that closes a winning position the moment a chosen price level is reached.Click the word to learn more, slippage tends to help, for the mirror reason. Either way, the math on a live account is rarely the math on a clean simulator.
This is one of the reasons Javlot does not accept backtests as evidence of fitness. Backtests can model slippage, but rarely the bad kind. The slippage that hits at four in the morning during a flash move on EUR/CHF is the slippage that quietly disappears from a simulation. The Vetted standard requires real broker fills, real slippage, real prices. Otherwise it is fiction.