MC-68015 is NOT a duplicate of this issue. Please remove it from the duplicate list. Why? Because MC-68015 is talking about a "glitch" where you can go past the world border at x/z 29999984, which is intended. However, I am talking about the invisible barrier at x/z 30000000, not the one at x/z 29999984, which is probably a bug.

Description:
There is an invisible barrier at x/z 30000000. You can't go past it, even in spectator mode. However, you can get past it by using relative coordinates in the /tp command.

Steps to reproduce:
Use the classic superflat world to demonstrate this bug easier.

1. Press F3 to look at the coordinates.
2. Type this command: /tp 29999999 4 29999999
3. Now stay there. Type /tp ~3 ~ ~3
4. You got past the invisible barrier. Minecraft freezes.

The next time you load the world, the "Illegal position" message displays.