If a learner makes a dangerous mistake, your first priority is ensuring safety. If necessary, calmly but firmly tell them to stop the car when it's safe to do so. Once you're stationary, take a moment to collect yourselves. Then, discuss what happened, why it was dangerous, and how to avoid similar situations in the future. Use this as a learning opportunity rather than a reason for criticism. If the mistake has shaken either of you, consider ending the lesson and resuming another day. Always emphasize that safety is more important than any driving maneuver.