When mentioning slippage most traders only think of negative slippage, where the price they receive is worse than the one they were attempting to buy at. However positive slippage also occurs and is actually quite common with limit orders. Positive slippage is when you receive a price that is better than the one you were attempting to buy at. For example, you might be buying GBP/USD with a limit order at 1.1965, but the order actually gets executed at 1.1962 thus getting you into the trade at a price that is 3 pips better than your order. That is positive slippage.