I'm sure this has been reported several times. If you make one mistake at a unit test which may take up to 20-30 minutes to complete, the punishment is completely unproportional and is very discouraging.

Usually this happens over small oversights and dumb mistakes. It can be soulcrushing to make an accidental mistake, knowing that you've just wasted a lot of time. No matter how you perform on the following questions will remedy this one mistake.

I suggest that unit tests gives you a "second wind" - a question at the end from the same category you answered wrong on, giving you a second chance to score points on the category. This would make the unit tests much fairer to the user - giving them the benefit of the doubt - as well as making the punishment for mistakes more proportional. 

I'm sure that this, or a similar solution, would make Khan Academy both more fun, rewarding and motivational for the learners on this site.