Abusing HTTP Status Codes to Expose Private Information
grepular.com/Abusing_HTTP_Status_Codes_to_Expose_Private_Information
From the page
Saved with the link. Notes from 2007 to 2016 are mostly the page's own words, so this is probably an excerpt.
Ignoring the privacy implications for a second, as a website developer, you might like to know if your visitors are logged into GMail; you could use that information to automatically fill the email fields in your forms with "@gmail.com"... Perhaps you might want to make your Facebook "like" buttons more prominent if you can tell your visitor is logged into Facebook at the moment? Here's how I achieve this: