GDM Login Issues
A known bug for 2 exec terms, all desktops experience an intermittent issue with logins. During username entry, the field will frequently erase itself while still adding characters. Even if you successfully enter your username, upon submitting you are usually rejected before the password field can even appear. In the rare case the password field does appear, entering your password will still result in rejection. This seems to be an evolution of an old bug, where you have to enter your username twice to log in.
The current room fix is to use another machine, or reboot. There seems to be no relation between machine uptime and experiencing this bug.
#37 (closed) was filed partially in response to this bug
bug has persisted across multiple Fedora versions