Fast user switching is not monkey-proof. With my laptop running Fedora 20 and GNOME 3.12, I can very easily get into situations where gdm is "stuck" with its gray background and no user/password prompt, no "lock screen shield", etc.

The way to reproduce this is to have two system user accounts, and to repeatedly try to "switch user" between them. You can also try doing a "logout" from the 2nd account for fun, as it can also trigger the bug sometimes, IIRC. Even "normal" use (ie not nekohayo "torture testing"), where you only switch accounts once or twice per day, can get you into this deadlock situation.

Sometimes switching VTs (ctrl+alt+F1/F2/F3/etc.) allows you to go back to the previous user account (in which case you may see the password prompt and then the screen shield falling down), but not always. I recall having had to resort to complete reboots, more than once.

I was wondering if this was a GNOME Shell bug or not (since it manages part of the unlocking/switching stuff these days, I think?), was told that this is actually a GDM bug.