I've got a computer on my bench right now and I'm connected to it remotely. Basic maintenance/updates/etc so I just pull up the window every now and then and check progress. It's 6' away from me and I can see the screen if I turn around.

Twice now, I've pulled up the SC window and the screensaver ("Mystify", with the lines around the screen) is running. Wiggle the mouse...nothing; the lines keep moving.
So, I turned around to check -- the screensaver has turned off and I can see that the mouse is, indeed, moving around. But the screensaver is still running in the SC window! It's not locked up... it's still moving, looking perfectly normal.

I had to close the SC window and reconnect. I just duplicated this by pulling up the display properties and Previewing the screensaver. Same thing, it kept running the screensaver in the window but, over my shoulder, the desktop was fine.