Green Demon Challenge/YMMV


 * Breather Level: Certain Stars such as Cool Cool Mountain's Slip Sliding Away experience no significant change in difficulty beyond the presence of the Green Demon mushroom; others, like The Princess's Secret Slide, aren't that much harder with the mushroom around.
 * The entirety of Big Boo's Haunt is ridiculously easy once you successfully lose the Green Demon.
 * That One Level:
 * Big Penguin Race. Since you're guaranteed to die trying to start the race normally, one of the only ways to even have a chance is to successfully execute an absurdly precise set of movements so that you can "talk" to the penguin from a lower part of the course. (This video (from 7:25 to 17:15) perfectly demonstrates the lack of room for error you'll have.) Even then, you can't go too low, or else you'll be so far ahead that the penguin's Rubber Band AI will literally cause her to rocket off the slide and get stuck below the stage and render the race Unwinnable for that attempt. And you can't slow down too much to let her catch up, since the Green Demon will be on your back the entire way through.
 * The 100-Coin Star of Shifting Sand Land. The main problem is that you're required to enter the Pyramid sub-level in order to pull this off, requiring you to learn both parts of the level. But it's the second "half" that turns the endeavor into a brutal Marathon Level, where you can't afford a single mistake in the Pyramid trying to collect those last 25-35 coins, lest you be forced to restart the world completely. Better hope you don't get stuck in the sand!