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Pacman installed gnome 3 when I updated last night, but I didn't get a chance to reboot to see it until this morning. When I did I got into gdm fine, but trying to log in would just get me a dialog that said something like Failed to load session "gnome". I was able to correct this by switching to a tty, plugging in an ethernet cable to get onto the network without networkmanager, and then installing gnome-shell with pacman. I suspect that I was configured to boot into gnome-shell when I should have been configured to boot into gnome-fallback. I'm posting this here in case anyone else has this problem.
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