This is the least likely thing to happen on your test. If the pupil is clearly out of their depth it will generally mean:

• The route is to busy or complex for them.
• The lack the foundation knowledge, understanding and skill that will allow them to have a realistic chance of successfully developing in the subject of todays lesson.
• The situation (perhaps with the examiner in the car) is just to much for them.

Whatever the reason a change of goal and focus for the lesson should be discussed and agreed with the pupil and responsibilities should be re-assessed before moving on the revised goal.