In FOREX, slippage occurs when the market/pending order is executed at a different price than requested or set in the order. It is more likely to occur when volatility is high, or around news events, or during times when a product is trading outside its peak market hours. In such a situation, the market depth (liquidity) is likely extremely low. Therefore, the trade may get triggered and executed at the first available price that may not be optimal.
Since slippage forms an inherent part of the trading environment, especially when trading under a market execution model, we have no control over the amount of slippage an order can get. Therefore, the execution of stop-loss, take profit, and pending orders are not guaranteed at requested prices. More information about slippage can be found here.