Mario Adventure/YMMV

Complacent Gaming Syndrome: The Fire Flower/Super Leaf tandem. Unlike many Super Mario World hacks where the combo's required to pass (for example) the dreaded ice block-flip block patterns, in Mario Adventure it's just another one of the many power-up combinations that can see you safely through various stages.
Demonic Spiders: Warkitu, a hacked version of Lakitu that throws quickly-exploding Bob-ombs. Since the B button is used for running as well as grabbing things, it's easy to die because of a Bob-omb you unwittingly caught.
Game Breaker: The Wand can kill anything, lets you stomp spiny enemies, and takes three hits to lose. It's a matter of honor how much you want to use it (especially in World 8), and it's rare.
Good Bad Bug:
- Mostly bad from a player standpoint; later levels end with two Boom-Booms, but jumping on both at the same time results in Mario getting hit. Although this is clearly a glitch, it works out for the challenge of the game — otherwise, it'd be trivial to exploit them into staying together in a clump. Possibly a Throw It In.
- It's also possible for Hammer Bros. to jump off the map.
That One Level/That One Boss: A few of the challenges in Desert Dares involve fighting multiple Boom-Booms. One has you fight four of them. It's very difficult to clear, unless you cheese it with a Super Star from your inventory.
Visual Effects of Awesome: The parallax scrolling lava background in the castle levels has to be seen to be believed, at least as far as NES graphics go.