What on earth just happened?

[Tom Horsley]

I ran yum update this morning, getting a gazillion updates.
I rebooted the system.

When it came back up, I couldn't log in. I'd get a KDM screen,
login, then immediately get the KDM screen again.

I tried to login with a "failsafe" session which gave me
(apparently) an xterm that printed black rectangles
for all characters :-).

I tried other sessions like xfce and gnome and got the
same KDM login screen popping back up after I typed
my password.

I got in a vterm as root, created a new user, and
logged in as that user with a default gnome session.
That seemed to work fine. I decided I should compare
what is running in the session with my original
user, so I logged out and logged in again as original
user, and now my original user login works fine.

Logging in once as a different user appears to have
completely fixed the system.

What the heck was that? (It will be interesting to
see if the same sequence happens when I install the
latest updates at home).