Can a limit order execute at a better price?

Yes, and it’s actually common rather than rare. A buy limit order can execute below your set price if the market gaps down and finds sellers at a lower level before reaching yours, and a sell limit order can execute above your set price if the market gaps up. The exchange’s matching engine fills at the best available price consistent with your limit, not exactly at your limit, so you’re never charged a worse price than what you specified, only ever an equal or better one. This is different from a stop-loss order, where the execution can sometimes be worse than the trigger price during fast moves, since a stop-loss converts into a market or limit order only after being triggered. You check the actual fill price on the contract note rather than assuming it matched your limit exactly, since a better fill is a genuine possibility, not an error.