I made a very simple edit (added (and then deleted before save, I think?) Template:Notice, and then added a section header) and after save, I got this:
Apparently an edit from a completely different wikipedia? I've never visited or seen that article before, so it was not a copy/paste error.
[Gabe, I cc'd you on this because Roan told me to; set priority to unbreak now because it's a brutal data corruption bug - edit lost and resulting article incomprehensible.]