Last night, I was trying out different screensavers. I selected and tested (i.e. clicked on the Preview button) the "Tunnel" saver.
The system locked up and would not respond to any keyboard activity. I could not even ssh into the machine from another.
When searching for "screensaver freeze" and "screensaver lock" before posting this, I found
Screen saver lock machine
in the Phoenix forum from yesterday (as well as several other older threads).
Normally, I just use the blank screen as a screensaver, to which I have returned. So I am just posting this as an FYI.