Can an executed stock-market order be cancelled?

No. Once an order is fully executed and matched with a counterparty on the exchange, it’s a completed trade and cannot be cancelled or reversed, since cancelling would mean unwinding a transaction another investor has already been matched into. What you can do is place an opposite order (sell what you just bought, or buy back what you just sold) to close out the position, but that’s a new trade with its own charges, not a cancellation of the original one. This is different from a pending or partially executed order, where the unfilled portion can still be cancelled before it matches. You double-check order details (quantity, price, buy or sell) before hitting confirm, since there’s no undo once the exchange has matched it.