This refers to when two or more people encounter each other in completely coincidental fashion. You might notice your old classmate from three countries away is now your waiter in a place you had no reason to expect them in, and you might say "wow, what a small world". You might notice two people who you know from completely different spheres miraculously know each other. You might recognize by chance that your penpal has made a cameo at a venue you're at.
But what was your most profoundly coincidental encounter?
(they are, in fact, two separate people. He became a youth pastor. I also remember he stopped bullying me in year 3 when he found out I could draw dinosaurs pretty well)