I was practicing my runs on level 3 when somthing weird happened. at the second checkpoint at the orange and blue box you need to jump on to continue the level, i hit the corner and clipped inside, i was able to walk around too, this might help optamize the run, i dont have any video proof, because it was just a practise

Which corner? The top or bottom?

top corner, sort of like the clip in the original smb1

maybe you try using a TAS to experiment