Hell knights and Barons should not be afraid of acid. Unless you removed their immunity. In which case they have good reason now ;).
The minigun description has a typo, Ait is supposed to be Air.
When you look at yourself using (M)ore, when wearing certain armors, the picture is sitting right on top of the text description. Same with spiders.
Sometimes when entering a level, monsters start off in a nice big lava/acid pool, ?and if completely surrounded by such liquid, they will just sit and burn instead of trying to to reach land? Also I don't know if its a bug or intentional, but with monster spawn levers, if the area is covered in hazardous liquid, they refuse to spawn.
I've found both a chainsaw and a normal BFG in the later levels of the 100 challenge, Intentional?
One other thing un-bug related, perhaps Spiders should have some minor resistance (if they don't already) to lava/acid, thanks to those metal legs they have.