The conversation after a mistake matters beyond the punishment attached to it.

What people learn from a mistake is decided in the conversation afterwards, not by the consequence attached to it. If the discussion is only about the penalty, the lesson becomes how to avoid being caught, and the actual cause stays exactly where it was. The useful part is finding out what the person believed at the moment they chose.

After something goes wrong, ask what it looked like from where they stood before you say what should have happened. Keep the correction separate from the account of it. Make the last thing said be about what you both want next time.