if you set a world border to a big size (like 30 blocks) and set the center past x/z 29999983 and then shrink it to a small size (like 1 block) and add a time that it will shrink then the world border will look rectangular, then weird like a 2 sided world border, but later it will ** crash your game.

Do NOT try this on your worlds or on your servers! It may PERMANENTLY crash your world/server!

Steps to reproduce:

1. Create a new world with creative game mode;
2. Type in chat "/tp 29999950 100 0" (this should teleport you at the edge of the world);
3. Type in chat "/worldborder set 30" (this should set the world border to a small, but not that small size);
4. Type in chat "/worldborder center 29999990 0" (this should center the world border past 29999983 x/z);
5. Type in chat "/worldborder set 1 20" (this should shrink the world border to 1 block in 20 seconds and crash the game);
6. Watch the world border shrink, become messed up and rectangular, and then messing up your game too and crashing it;

Because the attachment video exceeds maximum size limit, I attached it as a link:

https://filebin.net/pn8a5kqoj80seue0/cursed-world-border__attachment_for_minecraft_bugtracher_.mp4?t=whq3fl32