I was messing around in world 3-3 and I was at the platform with the last two koopas. I kicked one and jumped on the seesaw thingy. I jumped of and my head disappeared off screen the moment the shell disappeared down below. For some reason I took damage and became small mario. I tried to do it again but couldn't

This has been found before, but it hasn't really played much into runs. You can use it to go over the flagpole in a few stages, and there is a CE for it in 1-1. In terms of using it in an Any% or Warpless run though there haven't been any major uses found for it.

The only time ever where it was useful (for something important, not stupid categories lol) is in the ABC TAS where they use it at the end of 1-2 to save an A press