In the early 1980's I was the Vice Chair of ACM SIGGRAPH. I had to attend a lot of meetings for the organization as well as for conference planning. Around 1981 or so, we had one Executive Committe meeting in downtown San Francisco. For dinner, the 5 or 6 of us decided to go to Chinatown. None of us was a San Francisco expert, so we just walked along the main street (Grant Avenue?) till we agreed on a likely restaurant.
We walked in and sitting practically at the first table was a former student of mine, David B.! Mind you, at that time, I didn't even have that many "former students." I knew David was working for Silicon Graphics, but their office was down the peninsula, and not in downtown San Francisco. Moreover, he was having dinner with my former secretary (and I didn't have that many "former secretaries" either)! Turns out that she left Penn with him after he finished his degree -- and I didn't know that was the reason she quit her job.