In May 1996, a bunch of computer geniuses got together to write RFC1945, an informational document on the HTTP/1.0 standard¹ that was the foundation of protocol that defined the World Wide Web. Apparently, somebody noticed that referrer was spelled wrong², but by that time, it was already too late to change it. As a result, the HTTP specification indicates Referer, with 3 r’s, as the correct header, even today. I guess it shows that everyone can make mistakes.