Collision damage, ie: getting stomped, smacked, launched, etc into walls 1 hit k.o. I once had 4x resist with Omnioculus while invis, and 100 resilience, Gahlran jumped into me and I was launched into a wall and died. I wouldn't even mind being able to survive with 1-10hp from collision without spec'ing into dmg resist. Also not sure if there are any mods or any ways beside blocking with a sword that would negate some collision damage.
A similar thing happened to me in a Vox Obscura Master run the other day. I was at full health, and the Yellow Bar pison shot me through a wall, I was knocked backwards, I bumped into something behind me and was insta killed. I would have had a Flawless run too.