While minecraft is in fullscreen mode and a specific sequence is followed, Minecraft will sometimes mistakenly think that the Command key is held down. It can be reset by hitting the Command key again.

I'm still having some trouble reproducing this reliably, but it is reproducible sometimes. It happens with or without my external keyboard.

0. Launch Minecraft and connect to a server
1. Put minecraft in fullscreen mode
2. Use ctrl-left to move to another desktop (not cmd-tab)
3. Use cmd-tab to switch back to Minecraft (not ctrl-right)
4. Open chat
5. Type anything, then type "a"

The text you typed will be selected, showing that Minecraft still thinks the Command key is held down.

6. Hit the Command key.

Now text entry will work as normal.