Possibility of skipping warp stars
While investigating % flags and room IDs, I saw that the room that Kirby is considered to be in (according to EWRAM $020FE6, 16 bits) changes as soon as Kirby touches a warp star.
If it was possible to die while touching the warp star, you could go to the next area sooner.
I tried thoroughly but it didn't seem to be possible to touch an enemy and the warp star at the same time. I also tried with Bomb Kirby's self damage, doesn't work.
The flag for visiting the room the warp star goes to is still only set when it starts fading it.
Flags bug
When you unlock all switches, if you soft reset on a certain frame, the "have the all abilities room" flag is not set and therefore the all abilities room is not unlocked and cannot ever be unlocked on that savegame anymore.
It is not required to have it for 100%, though.
Double Hammer hit twitch vod
Dezero showed me a twitch vod of a possible double hammer hit on Dark Mind.
https://www.twitch.tv/videos/46416545
It is not fully understood, but the theory is that it happened because the mirror changed properties or hitbox, therefore accepting another hit earlier than normally possible.