Okay, I've faced the same problem as well with Mountain Lion OSX. I don't know whether you have the habit of mapping Ctrl+1,Ctrl+2,Ctrl+3... as keyboard shortcut for Switching Desktop ( or is that part of the default setting? ), but somehow got the extra Desktop disabled/ left unused. If you do... this is how I got out of it:
- Make sure Preference > Keyboard > Keyboard Shortcuts > Mission Control > Switch To Desktop 1,2,3,4 are unchecked
The tricky bits: if you are only having "Switch to Desktop 1" available but unchecked, you have to activate Desktop 2,3 to make the "Switch to Desktop 2", "Switch to Desktop 3" options *appear* for you to uncheck.
The OSX has a thing of invisibly intercepting the keystroke so there you go having Ctrl+1,2, etc doing nothing.
Similar principle applies had you mapped the keyboard shortcuts to other things before (I guess). Once it's done it's good to go.