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?
I was on a work trip back in the 80s that took me to one of the northern islands of Vanuatu. Our plane landed on a football field, that's how remote our destination was. After we set up camp, someone said they'd heard there was a teacher from New Zealand in the nearby village. Well I'm a New Zealander too, so off I went to meet her. Within the first few minutes we had worked out that not only were we originally from the same small town... she was my older brother's first girlfriend.
But actually because NZ has a small population and we all travel a lot, it's not as mad a coincidence as all that. It sometimes feels like we are all just a couple of degrees of separation from each other. "Oh you're from Oamaru? Do you know XY?" "Not really, but one of my cousins works for his sister, ZY."
I have family in Vanuatu and they run into scenarios like this a surprising amount. Maybe it has just the right demographic when it comes to relations.