2009/4/28 doug <drobertob@alice.it>

different databases during different editing processes. When this
happened I couldn't kill the program, the X-server, Ctrl-Alt-Del, etc.;
the screen, keyboard and mouse were frozen. I could only get out by
pulling the power supply.

Ctrl-Alt-Del normally does nothing. Originally Ctrl+Alt+Backspace kills the X server, but in ubuntu 9.04 it is changed to Ctrl+Alt+F12 I believe.
Anyway, when X freezes, you can normally do Ctrl+Alt+F1, log in a terminal and with top/ps kill the problematic program (or X), and relog in the X shell via Ctrl+Alt+F7