user:something (either a valid, or a non-existent user) returns all posts on SO.
I stumbled across a question where the OP's username was greyed out:
Curious as to if this was a bot or a banned user, or something else, I searched Meta for "Gnome"... but I didn't find anything relevant.
My side-question is: who's Gnome?
After that though, I searched Main for
Yes, yes, I know, bad search syntax... Still, it matches ALL posts on the site:
From Data Explorer:
From the search query: