I just upgraded to Gnome 3.14 and wanted to test new things, especially the new Pacman backend to PackageKit (through gnome-software). I just clicked on the "update and restart" notification and I couldn't boot anymore.
I remove the "quiet" kernel boot flag in Grub and saw that the offending line was "[OK] Reached System Update Target".
There was no timeout so my system just hanged like that for a dozen of minutes (coffee time).
No TTY was available.
I solved this by adding the "single" kernel boot flag in Grub, then log in as root, then remove the /system-update folder then reboot.