I was messing with beds until i found this.

How to reproduce:

1) Create a superflat world with the following preset:

minecraft:red_bed;minecraft:plains

You can replace the red bed with any other bed color

2) As the world generates you will notice something is wrong: The loading/generating squared is offcentered.

3)When you finally are in the world, all the beds are invisible, and in 20w22a the fog color used is the caves one while in 1.15.2 the fog color used is the surface one.

4)Break anything on the superflat world's surface, and the game should first lag like crazy, then after a few seconds, server crash.