Unexpected screensaver locked up Mac

I'm running Einstein and SETI on my Mac with BOINC Manager 4.43 and OSX 10.3.9. I have only joined Einstein since downloading and using BOINC Manager 4.43.

Earlier this afternoon, I came back to my Mac, which I'd left in BOINC screensaver mode but not showing any graphics because it was crunching Einstein, to find an Einstein screensaver there. This was a bit of a surprise as I understood it's not supposed to be working yet. The usual mouse clicking didn't wake up the Mac but just led to the "Spinning Beachball". I couldn't get rid of it by the usual escape keys so I had to reboot via the restart button.

Any ideas? I don't really want this to happen again, in case restarting messes up any files I might happen to have left open.