Two stories immediately come to mind.
1. Family vacation to Mexico City in 1977. For some reason we decided to go see a bullfight. Sitting in the areana, watching the bull fight, all of the sudden my father taps me on the shoulder and asks me if the group of people two rows down from us look familiar. I take a gander and say "yep, but I can't place them." My father takes out a business card, taps one of them one the shoulder and hands them the card. They all turn around and nearly fall out of their seats in a moment of "oh my God." Turns out, they were our former next door neighbors who had moved to Orlando two years earlier.
2. While doing a study abroad gig in CPH, I decide to go to DUB for part of spring break. I'm strolling outside of Trinity College about 11:30 pm (after the pubs had closed), mildly buzzed, with a heavy coat and full beard. Far from my usual college look in the States. All of the sudden, I get a tap on the shoulder. I turn around and I'm about to punch the offender, when I hear "hey are you Golftravelr???" As it turns out, it was a dear classmate of mine from college in the States who was studying in France. Needless to say, over the course of the next few days, many many Guiness were consumed.
Coincidentally, during episode #2 above, my classmate and I are in a pub and she had called home while in DUB and learned that Vanderbilt had won the NCAA basketball championship that evening. In one of those weird twists of fate, we were in a pub, having a few pints and heard some American accents. We go over to say hello, and it turns out that the four guys we introduced ourselves to all went to Vandy and had no idea that Vandy had won. Needless to say, when we informed them of the news, a serious party insued!
3. On a flight from LGA-MCI, I end up sitting next to my 6th grade teacher. Now that was weird to say the least!!