I've seen this in a number of situations under beta 4. The buttons in a dialog sheet cannot be cicked on--sometimes a keyboard equivalent works (typically Return and Escape)--but not always.
Example: System Preferences > Security & Privacy > General > Require password [n time] after sleep or screen saver begins. Changing this brings up a confirmation dialog sheet the buttons of which do not respond to a mouse click. System Preferences must be force-quit.