First of all, I'm speaking of webpage referral.
Second, let me quote Wikipedia:
The misspelling referer originated in the original proposal by computer "scientist" Phillip Hallam-Baker to incorporate the field into the HTTP specification. The misspelling was set in stone by the time of its incorporation into the standards document Request for Comments (RFC) 1945; document co-author Roy Fielding has remarked that neither "referrer" nor the misspelling "referer" were recognized by the standard Unix spell checker of the period. "Referer" has since become a widely used spelling in the industry when discussing HTTP referrers; usage of the misspelling is not universal, though, as the correct spelling of "referrer" is used in some web specifications such as the Document Object Model.
 Hallam-Baker, Phillip. "Re: Is Al Gore The Father of the Internet?" alt.folklore.computers, 2000-09-21
 Fielding, Roy. "Re: Referer: (sic)." HTTP-wg, 1995-03-09
It seems even the W3C isn't completely consistent:
- http://validator.w3.org/check?uri=referer - works
- http://validator.w3.org/check?uri=referrer - fails
- http://jigsaw.w3.org/css-validator/check?uri=referrer - works
What is the correct spelling to use (referer or referrer), either in general or specific situations?