This is one of my favourite maths jokes:
I used to do that... I was just shy of the best grade in my calculus 3 class, yet I still don't know my multiplication tables.
I don't know my multiplication tables either. But then I'm dyscalculic. I can't remember my own phone number if I don't have it written down on a scrap of paper in my wallet. I have no problems mentally retaining large portions of text, though.
It's too bad Dyscalculia wasn't an established diagnosis when I was in elementary school (and later on, as well), since my lack of math skills made me pretty much be labelled as an idiot by my teachers.