I have a mac mini and one user - mine - seems to freeze the system when left logged in.
It seems to be the result of the flurry screensaver.

When I return to the system the system is unresponsive, the screen is black except for the white outline cursor arrow and the mouse will move the cursor about. Keyboard presses simply bonk, even cmd-opt-esc. Using the keyboard (bluetooth apple) to adjust the volume seems to work fine.