forgetful people

I don't normally forget people I meet. I might not remember everyone's names but as far as faces go I'm pretty good. I'm that person who'll remind someone we've met before and they stare at me with their blank expression. That is until yesterday. I was facilitating some group discussions at a conference and a guy approached me and said, "Hey! How are you doing?" like he knew me. Thinking he just might be really friendly I said the same thing back. Then, he told me how he'd ran into a friend of mine (he named her by name) the night before. We made some more small talk all the while I wondered how the hell I knew him because it was now obvious we'd met before. I didn't find out until later that night when I saw the friend he'd mentioned and asked her about it. It turned out to be my old roommate's new roommate who I'd hung out with enough times that I should've remembered his face. Hopefully he didn't catch on to my nonrecognition.