Version:            (using KDE 4.2.0)
OS:                Linux
Installed from:    Ubuntu Packages

Sometimes the keyboard completely stops working within KDE while using ALT-Tab.  This is a very sporadic bug which I can't reproduce intentionally.  I'll be working with multiple kwrite windows, switching among them quickly, when the program selection window displayed by ALT-Tab will suddenly flicker and go away during the last ALT-Tab instead of remaining visible.

When that happens, no running application, including non-KDE programs, will receive keyboard events.  The desktop will not respond to the keyboard, either.  Mouse operations still work, and X11 will still respond to ALT-CONTROL-F? sequences (and probably others, though I haven't tried).  Going to text mode via ALT-CONTROL-F1 and back to X does not restore keyboard functionality within X, but the keyboard works normally while in text mode/not in X.

Since the mouse still works, I discovered that I can use any open applications's window menu (the ALT-F3 menu) to launch the "Configure Window Behavior" KDE control module, and enable (or disable) 3D effects to restore keyboard processing.  This problem occurs regardless of whether I have 3D effects enabled or not.  Using the KDE control module to toggle whichever mode I'm currently in restores keyboard response within X and all applications.