There is a lesson here. Idiomatic mistakes, at least the ones that stick, are not produced by the hoi polloi. They happen when people try to sound educated—or to be precise, when educated people try to sound more educated than they actually are. A little learning is a dangerous thing. You hear a word like vagaries or misnomer, you think it sounds impressive, you think you know what it means, and you deploy it the next chance you get. And then somebody who has less cultural capital than you, and who looks to you as an authority, picks it up and uses it in turn.
I don't think it's so much a hierarchy of authority, I think it's probably more like the children's game of telephone. Those of us who don't read dictionaries for fun — not that I know any people who actually do stuff like that — learn most of our new words through the context of conversation. The speaker doesn't have to be personally authoritative; the word just has to plausibly fit in the overall sense of the surrounding sentences. If I came across a word I didn't recognize, and I didn't feel like looking it up, I'd figure out what the speaker or author was getting at, and look for any vacant space of meaning, so to speak, where I could fit the new word in.
Speaking of pedantry, though, I found this amusing: I finished reading Dennis Baron's book A Better Pencil last week. Baron, a professor of English and Linguistics at the University of Illinois at Urbana-Champaign, had this to say on page 222:
Plus, as an editor of mine once told me ruefully, even printed books are never error-free: there's always some infelicity of style, misstated fact, or typo that has escaped the eagle eye of editor and proofer...
Followed by, as if to prove the very point, on page 241:
Nor should it come as news that all technologies of the word control access, or attempt to do so: the full force of the law will come down on anyone who tries to sneak a peak at the latest Harry Potter before its release date...
Nooo! They got the English and Linguistics professor, too! Damn it. Looks like I'm going to have to steel myself for a long, solitary guerrilla campaign on this front.