Having tried to sort out the categories of what people think of as rules of English usage, I thought I might make a start on the categories of what people call errors.
Make no mistake: there are errors of usage. You may encounter people who say that a native speaker of English cannot make an error of grammar, but I have been an editor for more than thirty years and know otherwise. But not everything that is called an error shapes up to be entirely wrong.
Outright errors: Problems with subject-verb agreement are common. Have a look at this sentence: "The group included more than 100 informants whom CIA officials concluded had been implicated in major crimes abroad." More on who/whom subsequently, but you can see here that whom is an error. The pronoun is the subject of had been implicated and therefore should be who.
But it's perhaps more interesting to look at the other classifications.
Spelling errors: Jan Freeman has pointed out at Throw Grammar From the Train that confusing it's and its is merely a spelling error. People know perfectly well what the difference is, but sometimes the wrong neuron fires. It's a misdemeanor, not a felony.
Confusion of homonyms: Some confusions of homophones and homographs may also be mere spelling errors, as in lead for led. But some come about when writers are plainly unaware of distinctions of meaning, as when people who have no experience with horses write free reign for the dead metaphor free rein.
Shifting ground: Every semester I feel obliged to go over lie and lay, and every semester my students' looks are blanker than the semester before. I suspect that laid is going to supplant lay and lain as past tense and past participle, and is in fact well along the way. But we're not quite there yet. Similarly, I tell my students that it's safer to just use who, except in the most formal contexts, because they only get into trouble with whom. And singular they is steadily gaining ground.*
Misjudgments: Many usages damned as errors are no more than miscalculation of the appropriate register. If you say in conversation that double negatives don't make you no nevermind, as was the case with Chaucer, I won't object, but I would edit such a construction out of your doctoral thesis. Balancing writer's purpose, subject, publication, occasion, and audience requires sophisticated judgments, and it is easy to go awry. Barrie England is doing the Lord's work at Caxton with a series of posts on "The Negative Canon," examining usages that are regularly condemned and trying to get at the whys and wherefores. Those posts will repay your attention.
Socially stigmatized usages: Ain't, I suppose, is the granddaddy, disparaged to little avail by generations of schoolmarms. But fashions change. Contact as a verb marked you as some kind of barbarian in the 1940s and 1950s, hopefully as a sentence adverb in the '70s and '80s, impact as a verb today. The zombie rules so beloved of the peeververein also offer many occasions to disparage.
What to do: For the peevers, all of these categories carry equal weight. Any error, even if it is not an error, affords them an opportunity to look down on you as a moron, a barbarian, an illiterate. It is only then that they can imagine themselves as Horatius at the bridge.
It is up to you to decide how much you want to cater to their prejudices.
Obviously, you want to clean up your prose. Great thoughts may be expressed in poor spelling, but you don't want your spelling to distract the reader from what you are saying. You don't want to give anyone an occasion to look down on you because you have confused homonyms. You want to make apt judgments about register. You would rather draw snarky remarks about what you said rather than the way you said it.
And this is why you need and want an editor to look over your work before publication.
* Reading A. Scott Berg's biography of Woodrow Wilson, I came across a blatant singular they in one of Wilson's letters. If someone as fussy and prissy about his usage as Woodrow Wilson didn't blink at it a century ago, why are you so concerned?
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