It happens sometimes. You click the “change course” button and directly after, you see you made an error. You forgot to check the “delay” box, entered a wrong number or entered a TWA in the COG box. If you’re quick you might still be able to keep the command from executing, and continue without performance loss.
When you click “send command”, your command is send to the server, and it can take a few seconds before your boat jumps and executes the command. Before that happens, the command is placed in your DC-list. If you refresh your DC-list directly after you send the command you can still see it in there. If you’re lucky you can select the command and cancel it before it executes.
This is something to practice. Try it a couple of times, see how long it takes before the command is executed and see how much time you need to cancel the command. If you can cancel a command within 3 seconds, you have about 70% chance of successfully cancelling a command. Not too bad.