Monsters become NPCs
I've had various monsters become NPCs, where clicking them brings me next to the them to chat (but the chat window doesn't pop) and the monster wouldn't attack me either.
The strangest happened yesterday where a named Boss had a ! over his head, like the NPCs do when you've completed a quest. Clicking him would take me to him as if to chat, and neither of us could attack the other.
Unfortunately I don't have a save game; exiting the game and restarting fixed the boss.