Jailer, Gaoler ... Goaler?

I was playing a little hooky yesterday from class preparation and reading a bit of Anthony Trollope's Barchester Towers. I came across the phrase "king, judge, or goaler." ("The temporal king, judge, or goaler, can work but on the body.") My first thought was that "goaler" was a typo for "gaoler," the British equivalent of "jailer," and I so emended the text in my mind.

My second thought was that maybe the reference was to some other function, and that perhaps I had been too hasty. So I looked in the online OED. Lo and behold, the spelling "goaler" was listed as one of the spelling variants of JAILOR, JAILER, GAOLER. A little more research via Google Print turned up a great many occurrences of the spelling GOALER with the meaning, and presumably the pronunciation of, "jailer." It goes back at least as far as variant readings in Shakespeare's Folios and seems to expire sometime in the late 19th century; Trollope was using a spelling that was on the way out.

Granted then the venerable pedigree of the spelling "goaler," does that make it any less of an error? Judging from the etymology of the word, the combination OA cannot be anything but a confused orthography for the more correct AO — and yet it was surprisingly popular and widespread.

From this I draw this lesson for textual studies, that it is necessary to distinguish between the proper ("correct") reading and the "correct" etymology. GOALER is the correct reading in Trollope, although the orthography is a poor representation (therefore "incorrect") of the pronunciation and the etymology of the word in general (which is why, no doubt, such a spelling gradually vanished in the presence of competing spellings).