Sometimes, a command can spontaneously gain the character "" at the end of the command. The character will not display, but it will be there, meaning that the last argument in the command will not work. The command will stay there if the command is copy/pasted, or if the NBT data is saved. This only happens in command blocks, but if copy/pasted into chat, it will still not display. To replicate the character not displaying, just copy and paste it into minecraft. I haven't yet figured out how to replicate it spontaneously appearing in commands, but it happens to me a lot.