In this question:
someone asked what the "$dx$" in an indefinite integral meant. In one of the answers, which was accepted, the answer began by saying "Formally, $dx$ does not mean anything".
This is incorrect. $dx$ can be interpreted precisely as a differential one-form or as a measure. I don't know if there are other valid interpretations. My point is that the answer begins with a bald falsehood.
Incredibly, the answer has 8 upvotes (as of 11:07 Eastern time Dec. 13, 2013). I would encourage anyone reading this who is a good mathematician to take a moment, read the answer, vote for this answer the way s/he thinks it deserves, and upvote his/her favorite answer, if s/he likes any of the given answers. I would especially encourage you to read Carl Mummert's excellent new answer to the question, which is infinitely better than the accepted one.
When I teach beginning calculus, I do not attempt to teach my students the theory of differential forms, but I do not lie to them either.
I downvoted the answer and left comments to the answer and under the question. Several others had done likewise. I upvoted three correct answers, including Carl Mummert's excellent answer from Dec. 13.
I also flagged the accepted answer, explaining why.
I would like to know if this is an appropriate use of a flag: an accepted answer containing a major conceptual error that is likely to mislead the OP. I am not talking about an answer that has a typo or something like that.
I have seen a question or two on meta related to my question but I don't recall any consensus being reached or any rules being quoted resolving the issue one way or the other.
This is the very first accepted answer I have seen in MSE containing such an egregious conceptual error, in several years of participation in MSE, and this is the only time I have even considered flagging an answer because it was wrong. If this is an acceptable use of a flag, I would probably do this once every two years or so.
NOTE: I found a similar question at http://meta.math.stackexchange.com/questions/4731/dont-flag-wrong-answers?rq=1 It contains a link to an "FAQ on flagging": http://meta.math.stackexchange.com/questions/4328/capture-the-flag-faq-on-flagging , which seems to have more to do with flagging questions than flagging answers.