I've had this happen a few times.
1. The first was New Year's Ever, 1982 (our 10th Anniversary was the day before). We were trying to get into the Emporium on Main Street, which was jammed. I heard someone calling me...by my maiden name, which I hadn't used in all of those ten years. It was DH's college roommate (who didn't know that we had gotten married!). They talked for a few minutes, and then he brought his wife over to introduce us to her. She was my Ob-Gyn's head nurse! She used her maiden name professionally, and I had no clue that they lived in the same area we did.
2. In November of 2002 I attended my professional convention in Atlantic City (The New Jersey State League of Municipalities). I was chatting with a lovely woman, and we both mentioned that we would be going on vacation right after Thanksgiving. In December of 2002, as I checked into All Star Music, she was the person in line ahead of me! We hadn't discussed where we were going, or exact dates! But wait...the next day I checked into Port Orleans Riverside--she was the person in line behind me!
Edited to add: I mentioned this thread to DH last night, and he says I forgot the funniest one! On this same trip (12/02), he didn't have enough vacation time from his new job, so he called in sick. We were walking through the Polynesian when we ran into....you guessed it...his boss! He also didn't have enough vacation time and had called in sick for the same days so that he could see his daughter in one of the cheerleading competitions.
3. In December of 2003 we took a family trip to WDW/
DCL. While we were on DCL's
Castaway Cay (a lovely island paradise) I saw my brother-in-law (who was travelling with us, along with his wife and their two kids) talking to a really great-looking woman, and acting like he knew her really well. He owns a limousine company, and she was one of his recent customers...he had taken her to the airport for her trip! All she told him was that she was going on a cruise.
Beth