Yesterday I discovered a little glitch in Super Mario 3D Land. Here's how to pull it off:
Go to W3-1. You'll spawn in front of a platform with a cannon on it and against the left and right of it a Note Block. As any form but Propeller Box Mario/Luigi (with a Propeller Box the player character rolls too slowly), roll into the Note Block so that the first roll is performed against the side of the Note Block and the second roll is performed in the corner between the platform's wall and the Note Block. The player character will clip into the Note Block! Because the space underneath is a little too low to even crouch in, crouching will make the character jitter up and down and crouch-jumps will not clip the character out of the Note Block, unlike standing jumps. To get out of the Note Block, simply walk or jump (when not crouching) out.
But why does this happen?
Well, if you roll into a Note Block or hit it any other way (but not an orange bouncy mushroom platform), its model will slightly shrink - but not only its model; also its hitbox. So during the short timespan the Note Block's hitbox is smaller than normally, the player can wedge themselves into the now-existing gap between the Note Block and the wall to clip inside the Note Block.
Well, if you roll into a Note Block or hit it any other way (but not an orange bouncy mushroom platform), its model will slightly shrink - but not only its model; also its hitbox. So during the short timespan the Note Block's hitbox is smaller than normally, the player can wedge themselves into the now-existing gap between the Note Block and the wall to clip inside the Note Block.
If I find more courses where this glitch is possible, I'll edit them into my post.