These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Goddamned Bats: Any dust-covered enemy that attacks successfully will break your combo. Since the last area of each map will usually have between 2 and 6 enemies, it greatly encourages planning where and when to use your combo nuke to avoid facing them without one.
“Stop Having Fun” Guys: In an unusual case the developers enforce it through their level progression. The only way to progress is to retry stages for perfect scores, and no reward for simply completing a level or making a single mistake. This forces players who'd prefer to just breeze through and see all the levels to keep trying the same ones long after they would otherwise have stopped.
Not to mention the level progression forces you to go for perfects on levels that are hard enough to survive, let alone clear without getting hit or breaking combo.
That One Level: Hideout. While it's not THE hardest level, it's set apart from other areas in the game for consisting of almost nothing but one-block platforms, walls, and ceilings. Every other inch of the level is covered by Spikes of Doom. But the real bastard move this level pulls is at the very end, where you land smack dab in the middle of two large enemies that, depending on their positioning, can attack you immediately without giving you a chance to react. And there isn't enough dust in the level to use your special attack, so you need to wail on them the old-fashioned way.