From bug 81210:
1) enter any username
2) start entering a password, press Caps Lock
3) enter a bad password
a "Password" field appears again, not "Username". Only way I know of to get
back to the "username" prompt is Ctrl-Alt-Backspace.
Cosmetic error. You can enter your username at one of the password dialogs.
I can't quite reproduce this with neither stable nor devel gdm tree.
fixed in rawhide at least.