I may have found a ground clip. Here's how it works.

If Mario is in a 1-block gap, you can still jump.

If you hold 'A' while in the air, Mario will stand up. You are not on the ground, so the game doesn't know to push Mario back. This means that his head can hit the ceiling.

Depending on the subpixels, he might be snapped up to floor, or he might clip through the ground.

MaibaRTA, can you find a spot where this is useful?

Not wanting to disappoint you but most people are aware of this kind of ground clip :v

This has been known for a very long time. A place in the vanilla run it used to be used was VOB2. This is now obseleted by the key clip.