Mainly caused by the shell hitting a very precise spot on the edge, causing it to move over a square and freeze. It's probably already been known, but since it's useless nobody really talks about it.
This is very old glitch see speedrun of TAS of SMB Warpless they uses these type of glitches
Something like this has been used for example in some of the "meme" categories to get extra lives or to get over the Flagpole in 1-1. HappyLee also used this in SMB2J to prevent Bowser from spawning in 4-4. I'm sure there are more applications of the "levitating Koopa"-thing.
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