Khan Academy has been a huge help in improving my SAT math score. However, I often find myself between mastery levels on a few specific topics. It is frustrating to be bumped down a level when I can already do those problems perfectly each time, only to be bumped down again when I advance and mess up on the same concept I did last time on the higher level. This "level-matching" system works well in many areas, but I wish there was a way to manually override having the difficulty of the problems decrease, especially if I've had to go back up to the same level 2+ times under ten minutes. I would like to drill the same kind of problem I got wrong without having to do five easier ones each time. 
Another idea - let us bookmark problems that we do during practice. I've had problems that I completely blanked out on, and when I read the explanation the previously alien concept clicked perfectly. It would be nice to be able to bookmark these (the task and the explanation) for future reference, or maybe even have a section where we could go practice only these problems. I currently do this by manually copying them down onto paper, but it would be super helpful if this feature could be integrated into the actual dashboard too.