might be a mac only problem, I don't know
When I go in a command block, if I accidentally tap the down arrow, Minecraft will insert an invisible character that can't be seen unless I copy / paste the command into a text editor.

For example, if I go into a command block, type "text", then press the down arrow four times, this will be the result:
text

This would not be a problem if it didn't mess up commands. If you type:
/summon Chicken ~ ~1 ~
Then it will return "Unknown command"

And on a side note, there is something else that seems to be a mac-only problem. Not a bug, but a problem. I would like to be able to turn off the control+click=right click thing in the settings somehow. thanks.