After upgrading from Lion to Mountain Lion, I find that the F6 key (which is really Fn+F6, as I have enabled "Use all F1, F2, etc. keys as standard function keys" under System Preferences:Keyboard) is not behaving as expected.
Under Lion, I was able to pass this key to an application (a bit complicated, but basically I pass it to Terminal which was passing it to a tmux session where I have configured F6 to switch to window 6). But now after the upgrade, when I press F6 (or Fn+F6 for that matter) I am switched to a different desktop! If I press it again, all the windows on that desktop disappear. A third press causes the windows to reappear, and after that it just seems to toggle visibility of windows on my alternate desktop.
I have looked at Keyboard Shortcuts in System Preferences, but nothing appears to use the F6 key, and I haven't found anything in Apple's documentation which talks about what the actual function keys are supposed to do – only what the Icons do which is different, such as adjusting volume or screen brightness. That is not what I am interested in, as I am using the Fn+ equivalent.
I am using an iMac with a fairly recent Bluetooth keyboard, and there is no "Num Lock" equivalent for F6 as apparently some keyboards have.
Best Answer
If you upgrade to Mac OS X 10.8.4 Mountain Lion and also have Parallels Desktop installed, you will be required to upgrade the latter to version 8. This new version by default uses the F6 key to "Show and Hide the Parallels Desktop". To disable this, use the menus "Parallels Desktop" : "Preferences", and click on the "System Shortcuts" tab. to show the above preference, which can then be unchecked or changed to a different shortcut key.