When you get a friend's request from someone that you have no idea where you know them from. The worst part is you have mutual friends from work and school! You post messages on each other's wall and they never know you have no clue as to how you know them.
Tom: who the heck is this requesting me as a friend?
Tom: I have no clue who this is.
Tom: Crap, I have a case of Facebook Alzheimer's