Just had a really weird bug with Screen Savers in OSX after upgrading to 10.9.2. I use hot corners to enable the screen saver (and thus lock the screen), but this stopped working after the update. Quick Google suggested it wasn’t actually the hot corners not working, but the screen saver. Further Googling uncovered it was something weird to do with the “Start after” setting. I had this set to “Never“. Changing this to “1 Minute” caused my hot corners to start working again. Changing it back to “Never” and everything was still fine. Most odd.