For some reason, one of my (seriously outdated) Kubuntu installations has the nasty habit of not locking the screen when pressing Ctrl+Alt+L. Not always, though. It seems to do this only when I'm in a hurry and need to quickly lock my PC before walking away. This happens often enough to be annoying, but not so frequently as to bother me enough to look for a proper solution.
Instead of looking for a proper solution, trying to determine what's stealing the focus of the Ctrl+Alt+L key command, I just settled for an easier workaround: lock the screen from the command line. I use the terminal most of the time anyway, so why not just use it to lock the screen as well?
The magic incantation is easy, if a bit cryptic at first:
qdbus org.freedesktop.ScreenSaver /ScreenSaver Lock
"qdbus" is a broadcasting service for KDE (Qt, actually). This command basically tells the screen saver service to lock the screen. Works every time, and with an alias in my bashrc, I don't need to remember that horribly long command. Now I only need to determine if my computer detecting when I'm in a hurry is a sign of sentience, and whether this is a threat to mankind. Will report soon.