When typing out commands in a command block, everything is fine at first. When you exit the command block and look in it again, some parts or all of it has been replaced with hashtags.

It doesn't happen all the time but it's incredibly frustrating as it cannot be fixed. It can go unnoticed and break a lot of things when making chains that use more than one command block. I've tried retyping the command and moving the block to a different area but it won't work.