So I've only seen 2 of these in my minecraft time, but, what these are is when you put a command like clone or fill inside it, when you use the command, fill commands won't work and clone commands will just completely break and looked cursed, thats where i named it, I've seen only two, as i said so these things are very rare and weird. also this works in 1.14.4, but for some reason bugs.mojang.com decides the bug can only be for 1.16.1 and 1.16.2 release candidate 2