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.
 * Paranoia Fuel: If you manage to lose the Green Demon, you'd better keep an eye out after, because there's no telling if or when it'll get free...
 * 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. On top of that, the Pyramid is fairly vertical and contained, the coins are spread throughout the level, and the majority of the floor in the center is made of speed-reducing sand, including pockets of quicksand. And the Grinders and Amps within the stage can stop you completely in your tracks if you misstep, which is a virtual death sentence. This turns the second "half" 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.
 * Tall, Tall Mountain's Mystery of the Monkey Cage. The stage itself is already a bit on harder side due to being mostly vertical and centered around the mountain itself, meaning there isn't as many opportunities to ditch the Green Demon, but this star seems like a reasonable one to attempt... until you notice the mushroom can clip through most of the mountain walls. And catching the monkey requires you to sit through two dialogue boxes. There is a place or two where you can trap it, but good luck figuring those out!
 * The entirety of Tick Tock Clock. It's a highly vertical stage like Tall, Tall Mountain, and it's enclosed like the Pyramid, with the stage comprised of a set of scattered platforms that aren't exactly conducive to putting space between you and the Demon without running several laps - and the Bottomless Pit the stage is set over also means the chances of a single blunder killing you are incredibly high. And there's plenty of other hazards such as Amps and Heave-Hos that're more than willing to contribute to the death count. Even in a best case scenario where a fall doesn't kill you or leave you a sitting duck for the Green Demon, you'll still have fallen several feet and may have to redo a significant part of the level. Oh, and before we forget, this is with everything frozen due to the stage's gimmick. God help you if you try this with everything moving...
 * The worst of the stars by far are Stomp on the Thwomp, where you have to make your way up through the entire stage with even less room for error than you had previously to reach the star above the titular Thwomp, and the 100-coin Star, which is a Marathon Level like no other that also forces you to contend with the awkward and scattered placement of the coins. If you plan to tackle this set of Stars, you'd better get that Control Stick smoking.