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I have a MacBook Pro running OS X Lion 10.7 and somehow I changed the behavior for the Fn key.

Whenever pressed, it shows me the desktop.

It's driving me crazy because some applications require a combination of the Fn key plus something else, so whenever I press the combination it still shows the desktop.

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Have you installed any apps that change keyboard behavior? – slhck Aug 7 '12 at 16:30
Not that I can remember... I'm almost 99% sure that I have not, I usually don't install programs to tweak my system. – MikeGA Aug 7 '12 at 20:55

I recently had the same problem. I got rid of this option by going to System Preferences > Misson Control. Where it says 'Show Desktop' it will say a key on the keyboard. Make sure it says 'fn' unless it does already. Then change it to '-' and this option will be gone :)

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I tried your suggestion but it doesn't allow me to change it to a dash '-' my computer just beeps, I can add a different shortcut but the 'fn' key is still showing me the desktop!! – MikeGA Aug 7 '12 at 20:53

OS X El Capitan 10.11

I found it, after years of searching.

System Preferences> Mission Control> Show desktop> Drop down menu change "Fn" to "-".

This has been the most annoying, niggling problem.

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My guess is that the Fn key was accidently set as a shortcut. In my experience with MacBooks, any "Key" related issues can be found here:

Apple Menu > System Prefrences... > Keyboard

Foe the Specific Shortcut you mentioned, I would check the "Show Desktop" in the "Keyboard Shortcuts" Menu.

Mine is set as F11. Make sure Fn does not appear there.

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It seems impossible to even set Fn as a single shortcut key in OS X. Have you tried it? – slhck Jul 31 '12 at 7:50
Ha. Excellent point... I went through all this trouble but did not test the actual "fn" button. Sorry about that. I'll look a little further, but I'm currently stumped. Did you try "Restore Defaults"? – Brad Jul 31 '12 at 17:43
It's not me who had the problem :) – slhck Jul 31 '12 at 18:00
Another excellent point (clearly a newbie at superuser...). MikeGA, I would suggest "Restore Defaults" to see if that helps. – Brad Jul 31 '12 at 18:04
I have upgraded to Mountain Lion hoping it would restore it but I still have the same problem, at the screenshot above I can change the show desktop shortcut to something else but It doesn't remove "fn" behavior, it still shows me the desktop!! I'm very frustrated!! – MikeGA Aug 1 '12 at 17:51

This was driving me nuts too but I managed to find a fix for it, it's similar to bradpfeiffers solution above but resets all keyboard defaults at the same time.

Go to System Preferences > Keyboard In there I reset everything to default though I am not sure this is absolutely necessary. I also unticked everything to do with Mission Control.

Go to Mission Control Preferences Untick all four options and change all four of the keyboard shortcuts to '-'

Then go back to Keyboard Preferences and reset defaults again.

Now my Fn key no longer shows the Desktop! I think it must have been a hangover from when I upgraded to 10.8 from 10.6.

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