SPISPOPD/YMMV

Accidental Innuendo: By Word of God, Sphinxster's name was meant to be a pun on "youngster", but it was only until after the release of Dr. Lunatic that Mike Hommel realised that it sounded dangerously close to "spinster".
Anticlimax Boss: Dr. Lunatic is this until he becomes a Super Duper Zombie.
Demonic Spiders: The Mutant Zombie in Supreme. It scatters poison all the time and regenerates its health whenever it is damaged. Its squash attack is also much faster than a regular Zombie's.
Even Better Sequel: Dr. Lunatic and Supreme are both this to SPISPOPD.
Evil Is Cool: Think about it. The protagonist in Dr. Lunatic is a bald man holding a hammer. He goes up against an Elvis-impersonating Mad Scientist who plays poker with a scorpion, two bears, a Sphinx and a Thing. All of them command armies of Cute Bruisers. Supreme one-ups this by giving the scorpion a gigantic bulldozer as an ally. In what way is evil not cool?
Funny Aneurysm Moment: The first game seems to be this to Mike Hommel. It's not surprising given that he makes family-friendly games under the Hamumu Software name.
Game Breaker: Missiles, as well as the Mines Rage attack. Even bosses will feel the pinch under a constant barrage of either.
Goddamned Bats: Scary Bats, Scarier Bats and Die Uberbat in Dr. Lunatic and Supreme. Even Die Uberbat, a miniboss, isn't really a big challenge if you have just a little bit of firepower, but he can summon Scary Bats to tackle you.
Growing the Beard: From SPISPOPD to Dr. Lunatic. The latter was one of the first games published under the Hamumu Software name, and it defined the company's culture for a full decade after its initial release.
Iconic Character, Forgotten Title: In Hamumu Software culture, Dr. Lunatic more or less overshadows the original title of the series, SPISPOPD.