This may relate to the recent auto-tabbing thread...
For using a proxy server to establish an internet connection, we have one checkbox for enabling the proxy, which makes active a textbox for the server and port plus another checkbox for authorization. Turning on the second checkbox enables username/password fields.
What should happen with the input focus when the checkboxes are turned on? Should it stay where the user last clicked, or should it move to the newly enabled textbox, or go somewhere else.