Command key keeps bringing up the Mission Control
OK, this one was just stupid.

In System Preferences ==> Mission Control, there is a section titled "Keyboard and Mouse Shortcuts".  For each of "Mission Control", "Application Windows", "Show Desktop", and "Show Dashboard", there is a dropdown where you can select which keyboard shortcut you want to use.

This is separate from System Preferences ==> Keyboard ==> Shortcuts ==> Mission Control.



In my case, it showed that "Mission Control" was mapped to &lt;Ctrl>&lt;Up-Arrow>.

I changed it to F1, then changed it back to &lt;Ctrl>&lt;Up-Arrow>, and now it works. Apparently, it had been mapped to  in a way that didn't update the GUI, and switching it to something different and then back fixed it.

I hate this kind of thing.