Login fails when preceded by incorrect password
WORKAROUND: After typing an incorrect password, click Cancel, then click your name, then enter your password again.
When on GDM, if you enter the password incorrectly and then enter the password correctly, the GDM GUI will disappear but you won't be logged into the session - you're left hanging on a purple screen with a cursor and have to hard shutdown.
Unfortunately I don't have a device to SSH into the session from so I'm not sure how to get the unabbreviated logs?
The Ubuntu bug is here.
Ubuntu's Sebastien Bacher reckoned this should be filed against GNOME Shell rather than GDM.