Why did my limit order get executed at market price?

Limit orders ensure that you will buy or sell stock at the specified price or better.

Your limit order will get executed as a market order if for your -

1. Buy limit order: limit price is more than the best offer price. If you place a buy limit order, your order will buy the stock at your limit price or a lesser price but not at a higher price.
2. Sell limit order: limit price is less than the best bid price. A sell limit order will sell the stock at your limit price or at a higher price but not at a lower price.

There may be two other reasons-

1. There was a sudden price movement in the market and your order wasn't able to be filled at your specified price before the price changed.
2. Your order wasn't set with "fill or kill" (FOK) or "immediate or cancel" (IOC) instructions, which can affect the execution of the order.