When a piston is used to push a solid block into a non-solid block touching the world boundary, the blocks z-fight with each other and when the chunk is reloaded, the non solid block becomes a ghost block which never goes away even when it's rolled back and the block is replaced. You can get kicked for flying by standing on it.

Watch the video to see how to recreate it and the effects. The download link is below.

https://www.mediafire.com/file/hha053gv6s8ei0o/Minecraft__1.17.1_-_Singleplayer_2021-09-20_21-26-44.mp4/file