If you Ground Pound into water so that Mario/Luigi can remain sitting underwater by holding DOWN (veeeery easily done in W6-2 ), you can make the plumber slide to the left or right on his butt by pressing the respective D-Pad direction, all the while holding DOWN as well. If you press a jump button while "sitting underwater", Mario/Luigi will skyrocket out of the water, with enough momentum to jump out of circa ten tiles deep water. This does not work if Shell Mario/Luigi for obvious reasons.

Similarly, if you hold DOWN after Ground Pounding quicksand, you can make Mario/Luigi slide around on his butt the same way as above.

If a Spiny returns to its walking state from its stunned (on back) state (very easily done in W4-5, which features both breakable bricks and roof Spinies), if it collides with the side of a breakable brick, the brick will break and the Spiny will turn around.

If you bump a block a Koopa is on and grab the shell while it's still flipping over, you'll hold a tilted Koopa shell. It's merely a graphical glitch, and if you throw or otherwise release the shell, it'll have its normal rotation again.

If Mega Mario/Luigi swims into a space narrower than his standing height (note: Mega Luigi's standing height it one tile higher than Mega Mario's), if the water level drops to below Mega Mario/Luigi, he'll keep swimming in midair due to being unable to stand.

Unlike with one-tile spaces between a solid floor and a solid ceiling, if Super/other two-tile-high Mario/Luigi uncrouches in a one-tile space between a solid ceiling and a semisolid floor, Mario/Luigi will clip through the floor.

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Non-glitch:

Could be useful in a jumpless hack that kills Mario/Luigi if he jumps: if you transition from standing on a ledge to hanging from it while an enemy you can bounce off, is right underneath Mario/Luigi, he'll bounce off of the enemy. This also applies if you automatically hang from a ledge after jumping onto it from a normal ground/bouncing off an enemy.

Some moar documentationz:

If you slide or Shell Dash into a collapsed Dry Bones/Super Dry Bones, only the body will get pwned at first. Once the skull attempts to return to the body, the game realizes the body is defeated, and the skull simply despawns.

When a cannon pipe bends left or right, its hitbox doesn't change, so any actors (that's what they're called, right?) affected by gravity/surfaces will remain on the pipe's hitbox, making them look like floating in midair.

If you Helicopter Jump off a Spin Block, the camera zooms out, right? But if Mario/Luigi's power-up state changes (either by collecting a power-up or by taking damage), the camera returns to normal zoom.

If a power-up spawned from the Touch Screen or from collecting 8 Red Coins falls through a column of bumpable blocks (like bricks), and you bump the bottommost block when the power-up is touching its hitbox, the power-up will get sent up through the blocks at warp speed. It stops going up when it either reaches empty space (after which it behaves normally) or gets pushed into a solid ceiling (destroying it).

If Mega Mario/Luigi shrinks back into Super Mario/Luigi, his feet remain at the same height. So if, after shrinking, the plumber's entire body is below a death border, either a natural one or one created by autoscroll or Challenge Mode, he dies. This can result in the humourous scenario where if the camera is scrolled up far enough and cannot come down again and Mega Mario/Luigi breaks the flagpole, during the Course Clear cutscene, he'll shrink back to Super size... and die!

If a Boo is placed in an area with music containing bahps, the bahps will permanently freeze the Boos in position.

If Mini Mario/Luigi is standing on a barrel and the water lowers, the barrel will remain floating unmovingly in midair. If the player jumps, the barrel will snap to the surface of the water; if the player transforms into any other form, the barrel will sink as if in water until it actually reaches water, after which the barrel will behave normally.

Not necessarily a glitch: if you shoot a fireball at a horizontal warp pipe just before entering it, while the "going through horizontal warp pipe" animation plays, the particle effects of the fireball making contact with the pipe will freeze. Then, near the end of the wipe, the particle effects unfreeze again. No glitch (I think), but it does look weird.

tcrf already mentions that 6-b has an extraneous goal flag a fair bit below any of the actual level--- it doesnt mention that this is also present in 5-3, in the exact same spot. it wouldnt surprise me if some of the levels in this game were edited off other ones (or something to that effect); relatedly, 6-b uses object bank for snailicorns despite not using them anywhere in the level. what level does use snailicorns, however? 5-3.
(also worth noting many levels use object banks for objects said levels dont actually use; i do believe this is known, but its worth bringing up)

I_enemy256_ncl.bin is a palette file containing 10 palettes, but only two of them are used: the first one (for the wallpaper changing toadhouse) and the second one (for the mvsl star). among the unused ones, the 9th is an early boo palette seemingly meant to be crossed out (credit to  TheBester7 for pointing this out. its pretty much the same as the final, barring being arranged differently), while the 10th is a duplicate of the blue coin palette (as far as i can tell. i suppose this could be used, but i find it unlikely and cant be arsed to check)--- the rest are pretty much all-black or all-magenta. EDIT: i already pointed some of this out earlier more tersely

 ItzTacos made a video some time ago showing off an unused actor which uses an unused graphic, A_hyouji_8bit_ncg.bin; from the video we can see it uses the jyotyu palette, which (assuming thats the palette its supposed to load and a hack wasnt done to get to work in absence of its actual palette) indicates the jyotyu palette looked quite different some time earlier in development (marios head there, for example, uses the palette lines for the blue shell and 2nd coin frame).

the bitmap for the score graphics has one for getting 10 points, but it seems to be unused (this was discussed a bit in nsmbcentral--- while certain actions in game give you 10 points, none seem to use the graphic).

also, while this is known by most nsmb hackers, i feel its worth pointing out that tile types like the 4x1 slope and blue shell brick are, as far as im aware, unused--- so i might as well write that here.

todo: verify springs react to moving stone blocks differently in the european version

OOOH, Nintendo blatantly edits existing courses?! They're violating one of our hack submission rules! Ban! "React differently"? How exactly reacting differently? How do they react in the American version? I'm asking this because I might be able to help. I own a vanilla European copy.

https://jul.rustedlogic.net/thread.php?pid=239890#239890

EDIT:

I can't read it right now, as the link throws the typical SSL connection error on my 3DS; I might read it tomorrow, on the laptop I also use. Nevermind,  Keeper was such a nice guy he edited his post to include the Jul conversation. So, if I interpret this correctly, any springboard will not get pushed by the moving stone blocks?

thats what it sounds like, yeah— or at least, for the US version

Well, I tested it on my European copy. I tested in W8-Tower 1, IIRC the only vanilla course to feature both springboards and moving stone blocks. The blocks did push the springboard on my version, but not if it's already up against a wall. Then it will clip inside the moving stone block and remain where it was - it won't get destroyed like power-ups.

i just checked how it works in the american version and it seems to be the same; ill probs have to check the level in the rom hack linked in the thread specifically

Good thing; it might be a glitch exclusive to that hack.

I tested on both the USA and European versions and the springboard seems to behave differently and a bit weird, in the European version, the springboards psysics are affected by the stone walls, but in the USA version sometimes they are and sometimes they aren't, also, I don't know if it's worth mentioning this, but the springboard gets crushed only if you throw it, here's an example comparison video:



And here's the level in case you want to take a peek
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My YouTube Channel

If you pause the game on the worldmaps of Worlds 2, 3 and post-Dry Bowser 8, close the lid and reopen it. Even though the game is still paused, the ambient background sounds of those worldmaps are heard now: wind blowing in World 2; the waves rustling in World 3 and the lava bubbling in World 8.

If you Shell Dash into Bowser Jr. while he's hiding in his shell, his bib will be gone during his "struggling-on-back" animation, but will be back once he goes into another animation, e.g. the defeat animation.

Internal file names heavily suggest that the Snailicorn was the Bully early in development.
Red Flying ? Blocks never contain a Mini Mushroom before one is obtained through another means.

I just read about the following in a thread: if you put a warp pipe in an area with 2D water from the jyotyu tileset, the warp pipes will become transparent. It was unwanted in that thread, but one could use it to their advantage too.