The BBC has an interesting story about this.
It is weird when you unexpectedly bump into someone you know when you're abroad, but it does happen quite often. It's a small world and if you move in similar circles, it's really not that unusual at all. It's happened to me several times.
Once I was transiting Frankfurt airport and met a guy I'd known from doing aid work elsewhere. Well, I guess Frankfurt is a busy hub for people travelling by air to various destinations in Europe.
Another time, I was sitting in Cairo airport and my 'cousin' (not a real cousin, but a sort of relative nonetheless) strolled by and said, "Hi, Kay." You could have picked me up off the floor after that surprise.
Obviously I go to places where I know my old friends hang out, but when I'm just passing through somewhere I've never been before, I don't usually expect to meet anyone I know.
Has it happened to you?