Victor Reppert did a post on undesigned coincidences that attracted some atheist halfwits:
I'm often struck by the chasm separating their intellectual pride from their intellectual ineptitude. They think just being an atheist adds 50 points to their IQ.
They also act as if the McGrews concocted the category of undesigned coincidences to prop up the rickety historicity of the Gospels. Seems to me, though, that the McGrews are simply applying a general principle to the special case of the Gospels.
For instance, when I was young, my maternal grandmother and all but one of her 9 children were still alive. My mother had childhood memories that sometimes left her with questions. Loose ends. So she'd ask her mother or one of her older siblings if they remembered the incident. They'd sometimes fill in crucial background details. Comparing independent memories of the same incident. Mutually supplementary memories. Things my mother found curious fell into place when her mother or an older sibling added some context to the incident.
None of this was planned in advance.
If you have two or more independent fictional stories, you wouldn't expect their narrative to interlock at any point. On the rare occasion that might occur, it would be sheer coincidence. Because two or more independent fictional stories aren't based on a common event, there's no reason their plots and characters would ever intersect.
Conversely, if two or more independent accounts cross over into each other with any frequency, that implies a common underlying event.