I've noticed that after clicking the login button or pressing ENTER key in the login form, there is nothing to inform the user that the login process has started. If the login takes a couple seconds (slow LDAP server for instance), the user might think they didn't actually initiate the login and retry it.
My suggestion would be to:
- Disable the Login button when the form has been submitted.
- Add a spinner or some kind of busy indicator to inform the user that their login is being processed.