ubuntu-sso-client 0.99.2-0ubuntu1, Ubuntu Maverick
1. Invoke the sign-on dialog. For example:
a. Clear your Ubuntu Single Sign-On credentials from your user account if necessary.
b. software-center --enable-buy
c. Choose "File" > "Reinstall Previous Purchases..."
2. Press Enter.
What happens: The dialog grows to display "This field is required" errors below both the e-mail address and password fields. <http://launchpadlibrarian.net/54362881/Screenshot.png>
What should happen: Nothing. The default button should be insensitive whenever either of the fields are empty.
It might be appropriate to respond to an attempted submission by displaying errors, *if* the form was very complex, so that it was not obvious which fields you had omitted. But that's not the case here: there are only two fields, so it's obvious why the button wouldn't be sensitive, and it's more pleasant to avoid the error.
Plus the fact that the button is sensitive might hint that actually a user is able to go ahead with the action (confusing, as they can't), however when they click the button, then the "This field is required" labels appear.
With the button insensitive, this hints that a user can't progress - the correct hint, as they need to fill in the entries.