Many usage debates, I think, are actually debates not just about the overtly contested words (e.g., "fulsome") but also about other words: "is," "should," "correct," "error." One commenter on the split infinitives thread, for instance, writes "It is generally bad form, to be sure, and it should be avoided." What I don't quite grasp here is what "is" and "should" mean here. Does that mean that the word is "bad form" in some objective sense, and should be avoided for some reasons that are in some sense obligatory? Or does it just mean "I don't like it, and I like writing that avoids it"?
Another commenter complains about people's use of "octopi" to mean "octopuses":
My favorite [faux language-police correction] was when I was corrected when referring to "the octopuses" by someone who told me that the correct plural was "octopi."
What makes this faux correction even better is that the split-infinitive police are correcting an acceptable option with another acceptable option. The octopus police are correcting an acceptable option with an error.
"Octopus" is not a Latin root, but a Greek one. A proper Greek plural would be "Look at those octopedes!" "Octopuses" is also correct. "Octopi" is not.
I certainly agree that "octopuses" is a perfectly legitimate plural for "octopus"; and I don't like "octopi," partly for the reasons that the commenter mentions.
But what exactly does "error" or "not" "correct" mean, when used about "octopi"? "Octopi" may well be bad Latin and bad Greek. But according to the English dictionaries that I've consulted, "octopi" is a fully standard English word. It may be grating to some people, because of its air of pedantry that proves to be etymologically ill-grounded pedantry. It may not represent "logical" etymology, though the life of the English language has often not been logical. Yet under what coherent and useful definition of "error" or "correct" can we condemn "octopi" as incorrect or erroneous, rather than merely inelegant?
UPDATE: For those who want more sources, the online Oxford English Dictionary reports for "octopus," "Plural octopuses, octopi, (rare) octopodes." Two of the examples it gives, an 1834 edition of Cuvier's Animal Kingdom XII and a 1942 National Geographic article, use "octopi." My New Shorter Oxford lists "octopi" as well; so does my American Heritage (4th ed.); so does the Cambridge Advanced Learner's Dictionary.
While checking still more sources, I have found two contrary ones -- Garner's Modern American Usage calls use of "octopi" "mistaken," and the New Fowler's Modern English Usage says it is not "acceptable" and is "misconceived." Webster's Dictionary of Modern English Usage, on the other hand, doesn't condemn octopi. The majority (though not unanimous) view from the sources that I've consulted, then, supports octopi as an acceptable plural, and it's hard for me to see why the usage listed in the OED, the American Heritage, and the Cambridge would be "wrong."