I don't know whether this is really a bug, but I have noticed during a few of my games that I clearly get hit but the hitbox does not register it. This happened to me both as a paladin and a warlock during times when I was especially wary of my health going down on hard mode. I was nearly dead most of the time I noticed as I clearly should have died when a fireball from a trap or the mage mook's magic ball hit me but it doesn't register as a hit and I get out of the situation. I'm wondering whether it's some kind of a natural block mechanic or whether it's a glitch. If it really is a glitch, I would love to exploit the hell out of it, so I'm interested how it works! :p