Under what circumstances HttpContext.Current.Request.UrlReferrer is empty

The situations where this servervariable works include the following methods of a browser loading a URL:

• Clicking on a straight HTML <a href> link;
• Submitting a form, using POST or GET, from a Submit button, <input type=image> or Client-side script (Form.submit ())

The situations where it doesn ' t work:

• Clicking on a favorite, history, or the recently-typed URLs list;
• Clicking on ' Home ' in IE's toolbar, or an item in IE ' Links ' toolbar;
• Using Location.href or Location.replace () in Client-side jscript/javascript/vbscript;
• Using Hiermenus (details);
• Typing the URL directly in the browser and hitting Enter or clicking ' Go ';
• Launching a clickable URL from an e-mail or MS Office document;
• Using Response.redirect/server.transfer;
• Using Response.AddHeader (302) or <meta http-equiv=refresh> to redirect;
• Loading the URL with XML (see article #2173);
• Misspelling the variable name (many people assume http_referrer).

Under what circumstances HttpContext.Current.Request.UrlReferrer is empty