I had a problem because when the update manager notified me about an update (manjaro, keyring) I did it but then my mirror was not in sync so that the update manager complained about some conflicts and missing packages.
Because the first step of the update was done (removing some symlinks related to graphic stack) while the second part was not done, the system hang and I wasn't able to log in because it continued to crash reloading the login manager (gnome - gdm).
Changed manually the mirror from TTY and then upgraded from pacman, once all updates were installed my system returned working again.
My second computer which was updated the next day had no issues, probably because the mirrors were up and synced at the time...
I think this is a common issues and is going to happen again, it would be good to implement some checkin into the update manager to make sure the mirror is synced before attempting an upgrade. Also I don't understand why after the update my mirror list got replaced without running a new rank (pacman-mirrors), would be nice to have the original mirror preserved or some kind of hook to execute the ranking again.