Comment 11 for bug 173861

Same happened here, but I found out that ANY newly font will cause this problem.
For example, I was using anorexia, one of artwiz fonts (available by apt-get) for Orage Clock in xfce-panel.
(For those who don't know, it's the clock application for Xubuntu, and I changed the font of time display)

After updating, xfce-panel just did not start on bootup, so for an end-user, the computer is completely broken.
(nothing can be done in X)
I had to delete my ~/.config, and took me a while to figure out why this happened.

So, from what I tried,
artwiz fonts
Gulim.ttc, Batang.ttc (Korean fonts)
cause this problem, but there were absolutely no problems before doing an update.