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.