“Hypercorrection” is the tendency to take a grammatical rule (sometimes a real rule, sometimes imaginary) and extend it via a mistaken analogy. I’m sure you’ve encountered this. It’s behind the idea of changing “doubtless” (a perfectly good word) to “doubtlessly” (not a word), and when you have a word like “ignoramus,” pluralizing it as “ignorami” (doing that satirically doesn’t count). But my candidate for the most common hypercorrections involve “who” and “whom.”
The use of “who” versus “whom” is pretty controversial, probably because there are a couple different issues at play. First of all, the rule itself — to the extent that there is a rule — is fairly arcane and depends on “nominative cases” and the like. Issues that for the most part nobody is particularly aware of or particularly cares about. The second issue at play is one that even grammarians often ignore: register. Register is matching your language to the context and subject at hand. It’s essentially “how formal you want to be” in a particular setting. For instance, you might say “good morning” to some people in some settings, and “yo” to others. You adapt the register of your language to fit.
The famous reference book The Elements of Style never mentions register; it just focuses on what it thinks the rules are — and for that matter, it doesn’t even follow its own rules very consistently. But register is quite real, and it’s important. After all, at least in English, the objectives of using language include being understood and accepted, “Following the rules” is in there, but it’s a much more flexible notion of “following rules” than if there happened to be a single authoritative guide. “The rules” are part of being understood, although you can stray pretty far from “normal speech” and still get your point across. But “the rules” might have even more to do with another goal: being accepted. Social groups typically converge on language use, and when one idiolect is accepted, it’s often adopted by the rest of the group. By the way, an “idiolect” is the set of habits of speech peculiar to an individual. It’s also a word that will keep you well removed from most social groups.
You’re probably waiting for the big reveal here that lays out the rules of “who” versus “whom” once and for all. Not gonna happen; I have no idea what the rule is. I think it’s probably something like use ‘who’ in all cases unless you find yourself in a situation where using ‘whom’ is not going to make you “…sound like a pompous twerp”, as David Marsh of the Guardian put it (see that? register again). Also don’t say “idiolect” if you can help it; definitely a pompous twerp sort of word.
