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.
Blue Mountain Zone 2 is technically "complete" — it was made as a proof-of-concept and intended to be a tiny level to bridge Red Volcano Zone 1 and Egg Rock Zone 1, giving players a moment to catch their breath. Just a bit before release, however, it was decided that Red Volcano should just skip straight to Egg Rock instead, leaving it unused. (It's still hidden in the game - for now.)
Flame Bait: Roleplay in a "Standard" server and players refusing to complete a level (even something as simple as Green Flower Zone) is a common occurrence.
Fridge Horror: When a Deton crashes into a wall, the animal is released. But... If it hits the player or another Deton, there is no sign of the animal inside.
Tails can easily fly through irritating bottomless pit sections... But even he has trouble in Egg Rock Zone.
Knuckles's gliding and climbing abilities can similarly be used to bypass a lot of platforming sections. It's actually worse with him, since the levels are rarely, if ever, designed against him, and there's no timer for either of his abilities.
Goddamned Bats: Sharps. They are blue hockey-puck things that turn red with a loud noise when they see you, and then they chase after you at very fast speeds while invincible, and the whole "get hurt and attack with temporary invincibility" thing won't work. They turn blue again after a while, but you're probably still collecting your spilled rings.
The Buzz enemies from Techno Hill Zone are also fairly annoying, with their tendencies to pop up out of nowhere and knock you off platforms.
Justified, in that it's supposed to be preceded by two more acts for pre-existing zones, another zone, and a possibly one-act zone. As it stands, the absence of those four zones makes it a jarring Difficulty Spike. The game is still in development, but priorities shifting to the multiplayer aspect put single player stages on low priority.
But even after that, ERZ will still be the hardest level in the game, unless it somehow turns into a Breather Level...
Word Of God has stated that ERZ is supposed to be that hard.