Having never encountered this in plain old Hexen, I'm assuming it to be a Korax bug.
I have on several occasions encountered a ghoul who, no matter how close I got to him, never attacked me, he just floated there paying no attention to me at all. If I killed him the game would CTD instantly. This has happened more frequently in the Death Kings levels, but a couple of times on the default maps.
Also experience a graphics glitch which predicts a CTD (with a numlump message): kill a ghoul and if there is a pile of metallic residue on the ground (I call it scrap metal, but appears to be some variation of the fighters gauntlets; I'll post a screenshot if you tell me how) where he falls, the game will CTD within a minute or so.
And just to mention another oddity, I recently encountered an invulnerable Afrit. I cannot kill him! Was playing as the fighter and stood there hacking and slashing at him with no effect at all, he just flew back and forth screeching and spitting fireballs at me. Finally left the area, returned much later and he was still there, and still invincible.