Sometimes in Mac Os X you will experienced this error, even you cannot use ‘command + tab’ function to move between applications. Luckily I have quicksilver installed, so I still can move or open another apps. There is a quick fix for this actually, just open terminal type this command : killall Dock. This will restart dock and fix the errors. If you start to experienced this several times then you need to do Repair Disk from Disk Utility when booted from the install DVD that comes with your mac package (note: this is different with Repair Disk Permissions!).