[solarengineer]

From https://en.wikipedia.org/wiki/HTTP_referer

“ The misspelling of referrer originated in the original proposal by computer scientist Phillip Hallam-Baker to incorporate the field into the HTTP specification.[4] The misspelling was set in stone by the time of its incorporation into the Request for Comments standards document 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.[5] "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 "referrer" is used in some web specifications such as the Document Object Model.”

[cmiles74]

I have to disagree that the misspelling "referer" has "become a widely used spelling" even with the qualification "in the industry when discussing the HTTP referrers", a qualification too specific to be useful, in my opinion. In my experience getting the spelling wrong leads to much frustration and annoyance to this day.