Mac OS X Expose failure

All of a sudden you cannot use Command-Tab to switch applications, expose keys and hot corners are doing nothing, and the dock is acting incorrectly. It's frustrating, and almost feels like your keyboard has been crippled, and multitasking comes to a screeching halt. But the fix is easy and this should not remain a permanent problem.

Multiple Monitors? Screensaver?
Depending on your answer to two questions will determine the cause and correction to your problem. Do you have more than one monitor? Did you recently wake your computer from its screensaver?
If yes then you may have also noticed when you try to Command-Tab or use expose commands your computer makes the error sound. For an as of yet undiagnosed problem OS X still thinks the screensaver is active. If you try to start the screensaver now it will fail, and furthermore opening the screensaver system preferences may or may not crash System Preferences itself. So what do you do?
Solution:
Put the computer to sleep, that's it. Once you're sure it's fully asleep (indicated by the power light going in and out, wake it again and all should be well.

Dock is unsafe for landing
If the above does not apply or the fix didn't work, you may be having trouble with your Dock (that handy bar that holds your active applications and favorite shortcuts). This too should be a quick fix, first try relaunching the Finder application. Hit Command-Option-Escape, select Finder and click Relaunch. Try your expose, Command-Tab, and Dock functions.

Leopard OS X 10.5
Still no luck? Then open up your downloads folder (Macintosh HD/Users/YOURNAME/Downloads) and check for any corrupt files, especially JPEGs. This is also an undiagnosed symptom, most likely it has to do with the 10.5 Dock displaying folder contents. So it tries to show the corrupted file on the folder stack on the Dock, and this brings the Dock and Finder to a halt.

If all else fails
If the above solutions still don't solve your problem, it's time to reboot, and most of you should know this. When it comes to little nagging things that don't destroy your work, it's best to just save everything you're doing and restart. This avoids the possibility of something worse occurring, and brings everything back up to speed and makes your operating system happy.


An ear to the ground
There may be other causes for this problem, it seems to show up in different ways in different versions of OS X. Some forum threads to keep an eye on if you didn't find a solution here or are curious are below.
macosxhints forums
macosxhints forums
macrumors forums
macrumors forums
macworld forums
apple discussions