Already Done For You: The Giant Spider (which might actually be a crab, or a dinosaur) in the first level is dead on arrival. And he apologizes for it.
Its name is Deceased Crab. And yes, that's where he got his name from.
The name still appears to be his own invention, as there's no other known source for it. The manual just erroneously calls it "GIANT SPIDER," and it isn't named in any other book, official walkthrough or magazine.
Doppelgänger Spin: Chameleon Man is a bloody face hiding in a room wallpapered in bloody faces along with three decoy bloody faces. Bloody faces.
Dreadful Musician: The punk rock torso who attacks with, and I'm quoting the manual here, "bad-playing guitar."
Eldritch Abomination: The final boss, a face who seems to be watching you with many many eyes, strewn throughout World 8
Eldritch Location: Technically, the whole game as you are on an alien planet. The levels progress with no rhyme or reason to them. The scenery often changes radically from happy to groteque, not only the first level but several of the boss rooms are more macabre in nature than the levels.
Flash of Pain: Bosses briefly turn white when hit. As for the regular enemies, it depends on the enemy type to determine which color it turns into when hit.
Getting Crap Past the Radar: The "level start" screen shows skeletons waist-deep in a lake of blood. No idea how that slipped past Nintendo's censors.
Not to mention when the very first level transforms into a hellish landscape after you pass by the big tree. This effect was supposed to happen in every level in the original game.
And they slipped in the word 'Hell', which wasn't allowed in the NES days.
Our Monsters Are Weird: Most of the game's entertainment value comes from the sheer lunacy of the enemies it throws at you. Burning Japanese schoolboys! Angelic sticks of dynamite! Joseph Merrick, for crap's sake!
Temple of Doom: Level 4 is set in an Egyptian tomb, complete with scorpions, sculptures of Anubis, hieroglyphs, etc. What it doesn't have are the game's two Egyptian-themed bosses, who appear in other levels...
Tennis Boss: All of them, unless you're fighting the bosses as Bert (which you really should be).
Unbuilt Trope: The change in Level 1 from relatively normal happy fun Nintendo to a darker version is oddly prescient of Eversion, right down to happy face blocks turning into nightmarish ones.
Unwinnable by Mistake: One nasty and cruel example, depending on if you watch the key icon or not: Killing 2 bosses in Round 7 gives you a key but killing the final one takes the key away, forcing you to restart the game.
Ventriloquism: The serpent dragon's speech bubble is pointing toward the right side of the screen, but it's on the left.