Have you ever run into a situation where you log into a site with SharePoint Designer to make a change or add a workflow.  Later, you need to log into the site as a different user (you're not logging in as the admin user all the time, are you?) but SPD never gives you the chance to provide different credentials.

How do you solve this?  You might try to clear you cache in IE, but that isn't going to work for you.  However, if you flush the workstation's DNS settings with the following command, the next time you log in you'll get the desired login prompt.

ipconfig /flushdns (read more about flushdns)