Happens to me a lot. Just yesterday, a woman yelled out to me and started talking to me like she knows me from, well forever. I very, very vaguely recalled her face, but had no idea who she was. What's worse is she got a cell phone call as I was making small talk (trying to coyly figure who the hell this is), and I was stuck trying to figure, "should I just walk away now," or wait till she finishes.
Embarassing situation.