...respawns.
Yep, that's right. I go into an area I'm unfamiliar with right after someone else has cleared it out. While I'm passing through, a whole bunch of mobs simultaneously respawn right where I am. Sometimes I'm able to escape, but there are sometimes bugs where the mobs don't retether but chase me halfway across azeroth until the toon is dead.
Not sure what can be done about this except to be super careful and suspicious when I go into what APPEARS to be an empty area and wait 10 minutes or so to see whether anything respawns there.