You may have experience this before, you are working away and suddenly Exposé, screen shot capture or hot corners stops working! It is an extremely frustrating bug I have experience for some time on Mac OS X, and up until recently only a reboot would remedy it. This is not something I have experienced on all Macs either, and unfortunately I don’t know what is causing it, but I have found a fix that’s made life a little easier.
I do a lot of screen captures using Command-Shift-4, and when it stops working while I have all kinds of files open, a reboot is a real productivity killer. I have tried killing the finder and dock using the terminal to no avail. One process that will fix this issue, although not ideal it’s better than a reboot.
Click the Apple icon in your top menu and select sleep. A few seconds later hit space bar; this seems to restore Exposé and screen capture functionality. This method should also fix disabled hot corners, although I have not tested it. Although putting your Mac into sleep mode and awaking it a few seconds later is much better than a reboot, a real fix would be much appreciated, so if you have a better solution or know what’s causing this I would love to hear about it in the comments.