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?
One from MMO. I got accepted to a guild and after some time I hear somewhat familiar voice in voice channel. Turned out it was guy from the same village I grew up in, I knew him and he was my sisters classmate.
One from RL. I went to second biggest city in my country and accidentally bumped to my friend (also elementary school classmate) at random street whom I haven't seen for years.