Using a Combat Test snapshot, the player is able to load old worlds in 21w08b. This should normally be impossible, but the game considers worlds made in Combat Test versions as ones that are able to be loaded.

To replicate this bug, make a world in a Combat Test version, generate a few chunks, then exit out. Open the world back up in 21w08b, and, you have an old world in 21w08b. You can fly around in creative to generate some 21w08b chunks, and go into spectator to view it from underground.

The game obviously isn't meant to handle this and things like commands will sometimes stop working entirely.

Also, if you are going to test this bug, you will want task manager to be open, as exiting the world causes the game to get stuck in the world saving screen. You'll need to close it with task manager.