Keyboard shortcuts are useful for quickly accessing common functions, such as saving files (Command-S), or quitting programs (Command-Q).
In addition to universal commands like saving and quitting, most applications will have unique hot keys for their own functions, even if they are the same key sequence. For instance, Command-I in TextEdit toggles between normal and italic text, but the same command in Preview brings up the inspector window.
Since hot keys can be very useful, Apple has a built-in option to set custom hot keys for a number of system and application functions. To do this, open the Keyboard system preferences and on the Keyboard Shortcuts tab select the service or function area in the left-hand box, then set up your custom hot keys in the right-hand box (see more on setting up custom hot keys here).
This option in OS X is very convenient; however, sometimes odd problems can happen when adjusting these settings. For instance, MacFixIt reader Bryan wrote in with a problem he was having with Mission Control hot-key assignments:
I have an older uni-body MacBook Pro with Lion installed. When I first installed Lion, I messed around with the Keyboard Shortcuts for Mission Control, Spaces, etc. I made a mistake somewhere and bound the right Command key to bring up Mission Control and I can't un-bind it. So basically, I can't use the right Command key for anything, ever. It's frustrating, I've tried restoring defaults in the Keyboard system preference, but it did not work. I assume reinstalling Lion might work, but I'd rather not.
If for some reason the hot keys are not changing when you try to edit them in one area, you can try editing them using the other options the OS provides. In this case, Mission Control's hot keys should be editable in the Keyboard system preferences. However, these same shortcuts are also available in the Mission Control system preferences and can likely be edited there with more success. In Bryan's case using these options should fix allow the Command key to be usable again, but if not then removing the preferences file for hotkeys would be needed (see below).
While these two areas are the ones that Apple provides for setting hot-key shortcuts, many times applications support options for setting hot-keys. Generally these changes are reflected only when the application is active, but if the application interfaces with the global hot-key settings than these can spill over and affect your work flow in other aspects of the systems. This can cause trouble if it overrides a common keystroke behavior. While OS X's tools for setting keys ensure that you cannot override common ones, some applications may not have this check and may allow you to do this.
MacFixIt reader "Notes" recently left a comment pertaining to such an issue:
A week or so ago I opened an application and created a keystroke in that application which was Command S and it was not to save anything. The application accepted the keystroke. It is now a few days later and I'm finding that I can no longer use the Command S keystroke to save anything. It is obvious that what I had created is overriding the Command S Save keystroke. My dilemma is that I have forgotten the program I created the keystroke in. Is there any way to undo what I've done without finding the offending application?
As with any other keyboard shortcut, the first step here is to go to the Keyboard system preferences and see if any application shortcuts are bound to the desired keystroke. If so then uncheck or delete them from the list, and hopefully doing this will restore the overridden shortcut.
If you try these methods for editing shortcuts, be they built-in functions or application-specific functions, and are not able to restore your hot-key shortcuts, then the last option is to clear out the settings file that contains the hot keys and have the system recreate it with default settings. Hot-key settings are account-specific, so first go to your user library in the Finder's Go menu (in OS X Lion you will need to hold the Option key to reveal the library). Then go to the Preferences folder and remove the file called "com.apple.symbolichotkeys.plist," followed by logging out and logging back in (or restarting the system). Doing this should clear all custom hot-key bindings for the current user account and allow the default ones to work as they should.