Dyscalculia. I haven't had a formal diagnosis, but have been aware of the problem since middle school. Prior to that, I just figured I sucked at math; turns out I understood the concepts and my work process was fine, but the numbers I wrote down were messed up--reversed, transposed wrong, etc. It wasn't until a middle school teacher forced us to redo all incorrect assignments/tests that I noticed this pattern in my work.
Dyslexia isn't really a problem for me; letters are easy, but numbers seem to get mixed up somewhere as they travel from my head to a piece of paper (or vice versa). It's an unfortunate problem for an engineering student to have, but I've managed to work around it. I'm somewhat slower than my peers when doing math (I force myself to work slowly & deliberately, and continually double-check what I've written), but it's not too bad. I also compensate by learning concepts so thoroughly that I don't have to spend time on an exam trying to remember how to do a problem, when I really need that time to keep an eye on my numbers & make sure I'm writing them down correctly.