41,972 subscribers a community for 3 years, 2 monthslast post 7 days ago [+]
The general idea is: two (or more Redditors) unexpectedly in the same place at the same time. If you don't already understand the reference, you actually might not want to look it up. Examples:
Someone posts a pic (or story), and someone else in the thread says "hey, I was there too!" and provides evidence to that effect.
Two people post a pic (or story) of the same thing in separate threads and you notice and link them both here.
A post talks about something or someone, and that person (or someone closely related) shows up in the comments. E.g., a post about the special effects in a movie, and the FX supervisor for that movie appears with details.
Redditor 1 posts a picture of someone/something, and Redditor 2 says "hey, that's the someone/something next door!"
No planned or faked coincidences