Spitting a null sprite, while standing on a brown platform, might cause 3 effects:
2) crash the game
3) some useful glitch, like the powerup incrementation
As usual, this post from Mister is a must-read. The platform was even used in the first game end glitch. However, the possibilities seem endless. I will use this post to link to movies with odd effects, that might be usefull in a TAS or to abuse in hacks.
Emulator used: lsnes rr2
Movies start from power on, but I used cheats to beat the first level. Load the files as savestates.
Turning Yoshi into other sprites
Reznor (not useful to beat the level, because he's not at slot 7 and Yoshi vanished)
Spawning a feather (without losing Yoshi)
Turning YI3 into an aquatic level (from the video of this post)
Movies will be added if new glitches are found...