This is a lame bug. I apologise in advance :-) Here goes. I assert that when I lock my screen with the "Lock Screen" menu option of GNOME, this means I don't want people looking at the contents of my screen. However some of the screensavers work by slowly distorting the existing contents of the screen so privacy can be breached. If I have not been clear _please_ drop me a note
if that's a problem, then don't pick a random screensaver. :)
OK, ok, I said it was a lame bug :-)