I have a paladin I mainly solo and decided to take the diehard feat when I took a level of fighter. Prior to getting the feat of course become incapacitated and have to roll to stabilize which didnt work alot so hence why I took diehard feat. But now since I took the feat I have never had a chance to see it work. Now since taking the feat never get to 0 to -9. I go straight from standing to DEAD every time.
First few times I thought bad luck of course but decided to do some testing on lower level quests. Let some mobs beat on me with no armor on, get hit, get close to 0hp then straight to dead each time. So you devs may wanna look into this, and before flamers start asking yes I bug reported it