During “against the grain” we opened the barn to find a troll. In the ensuing fight, two of our players used a damaging/staggering ult to hit the troll when it was close to its first regenerative phase, pushing it up a ledge between the windmill and the barn.
This resulted in the trolls regenerative phase glitching out. The health bar coloured for regeneration, but the troll, instead of falling down to regenerate, recovered from the stagger and started attacking. After beating down the regenerative health of the troll, we found that we could no longer damage it (and we tried for a solid 3 minutes). Likely not an incorrect display of health bar either, since we must’ve exhausted the trolls hp many times over (it still reacted to staggering ults and such, it just wouldn’t take any damage).
So we had an angry, unkillable troll with intact AI, and wiped due to spawning specials and hordes shortly after.