Sometimes being in a test/exam conditions makes people more vulnerable to panic and make errors. Even in written exams, my mind went blank over a few things and got out of the exam room and the things I forgot came back to me.
Not going to disagree with that, but too many people are too blasé about driving. Sometimes you're going to drive in "stressful" situations, e.g. late for that important meeting, kids are in the back screaming, girlfriend about to give birth, etc. That doesn't change the fact you're in charge of a 1-2 ton lump of metal travelling at high-speed. If you can't drive safely without panicking then you shouldn't be driving.