Ever had this happen?

So i was studying in the library today for my geography exam tomorrow when suddenly someone taps me on the shoulder. Suprised, i turn around and see this girl just having walked past me. She says hi and waves at me. I return the guesture and am filled with warm fuzzy feelings at the idea that someone is actually saying hi to me because my existance goes virtually unnoticed at my school. The only problem is this: the girl obviously seems to know me, and she looks very familiar, i just can't for the life of me remember who she is and how i know her! For her to have said hi, and actually remember me in doing so, we must have known each other somehow...i just can't remember how and know i feel horrible for not recognizing her now.

Keep running,
~ Kari