In a way this is sort of like "should I down vote ..." (which forms one solution to the issue) but is about specious answers (i.e. right sounding answers which upon closer examination are wrong) and how to deal with them.
I see 4 possible actions: 1) Put in a parallel answer, 2) Put in a parallel answer but refer to the other answers pointing out the flaws, 3) Comment in the original answer simply stating that things are wrong, 4) Comment in the original answer with all the details pointing out the flaws.
In a recent item I responded with 3), and poised the question in a way that if the "expert" knew what he was taking about they'd say "oops, ou're right that was silly of me let me fix that" and I also down-voted the response. The response I got was a snarky answer, and a retributional down-vote. Kinda funny actually.
The reason I choice the #3 approach is that like it or not, people look at the higher ranked answers as the correct answers, popularity seems to replace accuracy. So I'd rather have the original author give the best/nuanced answer rather than a wikipedia style answer. And I'd rather have the right answer than get points.
Another aspect is that it also allows one to figure out if the original answerer actually knew what they were talking about. In this case the specific domain knowledge was very weak.
If I'd answered like #1 - parallel answer then it probably won't see the light of day. a #2 Answer - rebutting the original answer in a another answer is messy and rude. and a #4 answer - cleaning up details in their answer also seems rude as well.
But it seems a downvote is also rude.
And this leads to a bigger question, should things that correct the "known" answer from the past, say items that are 2 years old, be done? I see people talking about a repository of knowledge, but the fact is someone searching for an answer only will look at the top rated one and not the new correction to that.
Maybe that is the nature of these types of systems, that they can tend towards a cargo cult type of body of knowledge.
So what should I have done?