Yes, any pending orders are subject to market situation, and slippage can occur.

Slippage is a common knowledge among investors, and it happens when there is no enough market liquidity to execute the order.

In this case, the order will try to find another available market price to execute itself.

When you set a pending order in your account, you may specify the price level you like to execute the order though, that would be the price where your broker will start executing the order at that price.

Pending order will not guarantee you the execution without slippage.

It is possible that your pending order gets executed at a difference than the price you have expected.

Slippage happens especially when:

• The trading volume is large
• The market volatility is high
• The market liquidity is thin

It is even possible that if the market price jumps to another price in a moment due to extremely high volatility, and you had your pending order level between the prices, your pending order gets executed at the next available price which is very far from the level you have set.

Each MT4 and MT5 broker has their slippage and execution policies stated in the official website.