This is really odd. I use my Photo Stream for my screen saver and of course I have it lock the screen after a certain amount of time. However, in Mavericks, if I start the screen saver, either with a hot corner or a time delay, the screen saver starts, but then a few moments later it stops! The desktop just comes right back. Thus, I'm unable to lock my screen without putting my MBP to sleep. Has anybody else experienced this? Any way to find out why the screen saver keeps disengaging? Or is this a bug that somebody can reproduce?