HTTP/1.1: Header Field Definitions: Did you ever wonder why the HTTP_REFERER header is missspelled? (You did know that “referrer” is actually spelled with two “r“‘s, right?)
It turns out that the misspelling harkens back to the HTTP spec, as linked above.
That link is to the 1.1 spec, but in a fit of curiosity, I went looking to see how far back the misspelling went. The earliest I found it was in RFC 1945 from June 1996. Can anyone document it earlier than this?