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 moved to a smaller city in South Korea in 2004 to teach English. A short while after I got there, I met a couple who were from a small town down the road from the small town I grew up in in Eastern Canada. Apparently we even went to the same small university (3000 students total) together and I somehow managed to never see them there.