Bugged Meeting Quest
I had Fire Shield active, and was fighting the second monster of a meeting quest. The first was dead. The monster hit me so hard that I died, but the monster died as well from Fire Shield damage.
The quest didn't register the monster's death, meaning that it didn't complete, but since the monster was in fact dead I couldn't kill it again to complete the quest. Eventually the quest timed out.
I think it did eventually respawn, but it was too late by then for the quest. I'm not 100% sure about that part, since I killed a named monster of the right type, but I don't remember the name.