An order close error 138 in Forex trading is a situation when a trade fails to be closed at the intended price. This can lead to big losses for the trader. This type of error usually happens when a market moves rapidly in an unexpected direction, creating a “slippage” in the market. This can cause the order to close at a different, and often less advantageous, price than the trader had originally intended. Therefore, it is important for traders to be aware of market changes to reduce the chance of this type of error occurring.