I have captured some prisoners, and two of them keep fighting each other. Neither is broken (one is neutral, the other content), but three times now they've fought each other until one is incapacitated. They're from the same faction, so it's not an issue with enemies.
Sounds like a bug. Can you share your save file?
Unfortunately I have since captured more and they seem to have stopped. I will do so if it happens again.
Tribe or pirate faction? Were they injured or did they have any other hediff at that time?
I had this happen earlier in my A11 colony with a prisoner that had been berserk earlier (don't have the save, sorry). I think it might be the bug that's been around for a while where people continue fights started when one was psychotic, even after they aren't broken anymore.
I searched trying to find reports of this old bug:
Apr 2014 forum thread (http://ludeon.com/forums/index.php?topic=2923.0), colonists, Tynan links to Mantis 641
Apr 2014 Mantis 0000641 (http://ludeon.com/mantis/view.php?id=641), colonists, Tynan says "I haven't been able to repro the bad behavior."
Sep 2014 Mantis 0001197 (https://ludeon.com/mantis/view.php?id=1197), prisoners, Rahjital says can reproduce it with a monkey and a prisoner, but not two prisoners
Dec 2014 forum thread (http://ludeon.com/forums/index.php?topic=8622.msg85430#msg85430), colonists
Jan 2015 forum thread (http://ludeon.com/forums/index.php?topic=9170.msg92725#msg92725), colonists
Feb 2015, on Mantis 1197 Tynan says "Tried Rah's repro, seems fixed now."
So, was one of the prisoners berserk (and fighting the other) at some point?
Sadly I don't have a save to contribute, but when I saw this bug I just wanted to mention that this happened with two of my prisoners as well. I had two tribals in a prison-cell, don't remember if they had been berserk to begin with. Might have been. I do remember that I had a Evil Ship lying on the map. I thought that that was the reason, that it drove them insane.
I investigated this bug extensively in-game today, andafter watching prisoners fight for hours I think I have repro conditions.
Involved
Pawns being hit accidentally.
Maybe involved
Prisoners healing from many small wounds at different rate, resulting in being on the brink of incap, and healing above it then being bashed and going below threshold.
Indeed, Axe and I have been going back and forth on Mantis on this and I think I've solved it.
If not we'll catch it in the next internal build.
Thanks!