> I'm all for descriptivist language, but there's a difference between a new word or meaning coming around because people widely use it, and a common error that does not become right just because it became too common.

There isn't. At least not in any objectively recognisable way. You (and I mean you specifically) can obviously look at a new use and say "this is a new meaning" or "no, this is an error", but that's a subjective judgement and others would think otherwise. It is as impossible as being given a copy of a movie and being asked, by only inspecting the movie, to tell if this was copied from a legal DVD or downloaded from a torrent.

Just look at the verb "to troll (on an internet forum)". It comes from the practise of trolling in fishing, where you tie a bait to a cord and drive your boat around, drawing the bait through the water and waiting for some fish to take it. A lot of people, however, connect it with the race Troll, thinking of it as the practise of a big, nasty creature coming out from under and bothering people. That's not helped by calling the people who do it "trolls". Was the original, fishing term wrong? Was the person who coined the name Troll for a race of fantasy creatures wrong? Were the people who connected trolling with Trolls and subsequently called trolls "trolls" wrong? For bonus points: if two words are written the same and pronounced the same, can they be different words, or are all their meanings different meanings of the same word?