Spelunky/YMMV

Adaptation Displacement: The commercial version is a lot more well known than the freeware version.
Demonic Spiders: Those yetis. Those chain-throwing, whip-invulnerable, blue-nippled yetis...
- The actual spiders, whose unpredictable jump patterns make them some of the most dangerous enemies in the game.
- And cultists who throw you around the same way as yetis. They also jump around very fast. At least they're vulnerable to your whip.
- Perhaps the most unexpectedly deadly enemy in video-game history (barring the carp): Frogs. Completely unpredictable jump patterns and timing, inexplicable ability to predict your next movement, and quick vertical movements that make them difficult to take down with the whip. You will die by frog at some point.
Ensemble Darkhorse: The Tunnel Man.
Game Breaker: Getting a jetpack greatly improves your chances of survival. If you can get a shotgun on top of that...
- Then there's the Scepter which fires instant kill projectiles that chase enemies and go through walls. Unfortunately, you can't break the game with that indefinitely.
- Playing as the Tunnel Man in the original lets you use his mattock, which never breaks. You can tear through terrain to your heart's content without having to resort to explosives. The trade-off? You can't attack in midair with it, you start with no ropes or bombs, and he has half the default HP than the other characters.
Goddamned Bats: Almost every enemy in the game. There's goddamned spiders, goddamned yetis, goddamned cavemen, goddamned frogs and so on. Bats themselves are quite slow and predictable, but because they spawn hanging from ceilings and are obviously capable of flight, they're hard to kill safely because they always seem to attack from just above where your whip hits the air.
Nausea Fuel: The Kapala may be bad enough on it's own, due to the fact the player will collect blood from various sources (humans, yetis, spiders, etc) and then drink it to regain health... however, blood is not the only thing the spelunker can collect with the cup. In the Temple level, the hero can actually collect mummy vomit in the Kapala, that's right, you as the player can camp out under a mummy as it continuously rains vomit on your head, and then drink said vomit to gain ludicrous amounts of health.
Non Sequitur Scene: When you beat a custom level, there is the Walrus Lady. There is no explanation. There can never be an explanation good enough.
That One Boss: All of them, but especially the Giant Golden Olmec Head.
Unfortunate Implications: Rescued damsels show up on the score screen under "loot".
- The "Kissing Booth" has a red light. This can be easily seen in dark levels.
- Hitting the Damsel in the Kissing Booth, accidentally or otherwise, prompts the shopkeeper to say, "Hey! Only I can do that!".
- Damsels taken from the Kissing Booth are apparently just as grateful as those rescued from the middle of a monster filled maze. Maybe those shopkeepers aren't such nice guys.
- The booth owners are willing to SELL them. Underground prostitution?