I have set up 2 command blocks to test wether i have at least 8 levels and a score of my custom objective "moshroom" set to 0. While testing i found out that the command generated an error "The entity UUID provided is in an invalid format". When rewriting the same command into another command block this output changed to "Found Donald" (Donald is my username) so i figured that the command was working. However the same command was still not working in the first command block. This strange bug affected not only one but several of my previously working commands who just now appeared to be broken.

Working command: /testfor @p[lm=8,score_moshroom=0]
Broken command: /testfor @p[lm=8,score_moshroom=0]

I have also attached 2 screenshots showing each command block (one working, one not working) with the same command in them. The one that is working is retyped by me, the one not working was simply copy-pasted from the one i got the error message from.

I could reproduce the bug several times just by re-activating the command blocks. Changing parts of the broken command does not fix the issue. Only completely rewriting the command will work.

Edit: Upon closer inspection there seems to be a special character at "lm=8" of the broken command. The character was not visible in game and thus broke the command.