I thought of some more! I'm honestly not making these up - they just keep popping into my head now I'm thinking about the subject.
At university in 1999, I was at a house party and got chatting to a guy who was a "townie" in the city, not a student. Seemed like a cool guy. Had a nice chat. Thought no more of it. Then, ten years later in a city in the Far East, I turned up at a "media networking" event, and ran into the same chap, who was working for a magazine out there!
This one isn't strictly mine, but it's a good one. My friend was hiking up a mountain in Borneo (as you do), and struck up a conversation with some guys on the summit. It emerged that one of them was the brother of a girl we went to uni with.