On a SharePoint application, I was using "Sign in as Another User", but the previous user's name showed on the site until I clicked Refresh at least once, sometimes more.

To fix this, I followed advice I found here: http://stackoverflow.com/questions/168946/iis-returning-old-user-names-t...

I went to the Local Security Settings (in Administrative Tools) and browsed to: "Local Policies --> Security Options" and picked "Interactive logon: Number of previous logons to cache (in case domain controller is not available)" and set that to 0. Then, I did an IIS Reset. Seems to have fixed it.