Firstly, let me define what an asymmetric slippage is officially (per the NFA). This is the practice of taking advantage of the market by placing orders a few notches below the market price. Here is an example:
Imagine that a buyer places a market sell order for EURUSD at 1.38585 and the order is executed at 1.38582
Similarly, a market buy order for EURUSD quoting at 1.38585 gets executed at 1.38588.

It indicates that the broker rips you off with asymmetric slippage if it occurs consistently. The usual explanation given to clients is that prices have changed between the time a client clicks his/her mouse and when a trade is actually executed. At least once we should see a better price if that is the case. According to statistics, 50% of the order should be executed as above, and 50% in the trader's favor. Nevertheless, the client always loses. If this is the case, you need to speak to your broker right away. Imagine millions of these transactions taking place every day. At the end of the day, these nickles and dimes add up to millions for forex brokers and liquidity providers. Have you experienced that with any brokers? Please share your experiences without mentioning names.