This is gonna be a long one to explain.
My hunter became seriously injured, red (mauled) chest and throat. He made it back to his room and laid down to rest. He healed a bit the next season and then was beaten back to his first state by an irate mayor after that.
At the turn of the next season, he suddenly starts saying "Cancels Rest: Interrupted by Giant Bat" This is odd. There IS a giant bat on the map but it's many z levels above him and wandering off. So I kill the bat. Still the same message. Not only this but this message is coming up hundreds of times in a second. If nothing is going on in particular I could watch thousands of these cancel rest messages pop up. By the time I ended this it numbered somewhere in the hundred-thousands and was cutting my FPS down by half.
So I lock him in his room in the hope of starving him. He seems to be completely stuck though, as three seasons pass without him stopping the Cancel Rest notification, getting hungry, thirsty, or changing his current mood of Ecstatic (mostly because he was not being beaten further).
I begin a process of creating a long dropshaft below him in the hopes that I can remove his bed and channel out the floor before someone moves him, and have him fall to his death.
The second I remove the bed, he bleeds to death, and it's all over. FPS goes up to normal.
I HAVE NO IDEA.
I doubt it's replicable. Seems sort of similar to error 000957 though.