Recently we have discovered a serious bug in Fedora 17, that is also present in Fedora 15 and 16 (I’m not sure about earlier releases). If you have your system updates applied automatically (which is enabled by default), you might break your system when shutting down/rebooting the computer. There is no safeguard that would tell you “please don’t turn off your computer now, system is just being updated”. No, we simply abort the update process in the middle. Horrible things may happen. Fortunately that doesn’t occur too often, but the possibility is there.
I can heartily recommend everyone to disable automatic updates until we have a proper fix ready. In GNOME just open a tool called Software settings and switch Automatically install to Nothing.
KDE guys can be happier, because they have disabled it by default. But again, I recommend to check the setting. You can find it in System settings -> Software management.
It is highly unlikely that a proper fix will land in Fedora 17 Final. We will probably just flip the option in GNOME to have nothing installed automatically by default. That is not a fix, just a way to minimize damage. A proper solution might be available no sooner than in Fedora 18. Until that time… I don’t recommend to enable this option.