So I was messing around in 18w47b, and I used a repeating command block to create an infinite summoner for experience bottles. Then I saved and quit.
I came back a bit later, and re-entered the world.
I find this (see screenshot).
I can't move except via /tp, but I can look around perfectly fine. Even gravity doesn't work!
Everything is invisible except the particles for the experience bottles. I can still hear them breaking as well, but I can't collect any exp.
I think this is a very serious bug that needs fixing soon!
Steps to reproduce:
1. Place down a repeating command block.
2. Change it to "Always Active".
3. Set the command to: /summon experience_bottle ~ ~3 ~
4. Stand on top of the command block.
5. Wait a few seconds.
6. Save and quit the game.
7. Re-open the world.

EDIT: The bug fixed itself after ten minutes or so, and the world loaded back in, but this is still a very important bug to fix.