I see people I think are probably friends-of-friends or someone I ought to know around, too. The most interesting case was at a show at MIT when I was really bothered by the sense that I should know the guy sitting in front of me. After the show, he was in the lobby, so I did the "This sounds weird, but I think I know you from somewhere" bit. He told me where he went to college - nope, where he worked - nope, then finally it clicked and I said, "Oh my g*d, you're Dan, and you played the trumpet in our high school band." At that point he remembered me too (french horn), and his girlfriend showed up from the restroom and asked me if it was true that you only had to dial 5 digits of a phone number in our home town (it was indeed true until about 1990 when you had to start using all 7). I hadn't seen this guy in about 12 years I think, and we weren't particularly close in HS, so who knows what will stick with you!