While I certainly think that the OP of the question Non trivial zeros of Riemann zeta function definitely abuses MO in a couple of ways, still I believe it is a good example for the question in the title.
There is one question here on meta addressing this, Dealing with possible incorrectness, and the accepted answer (by a moderator) suggests to edit into the answer your opinion and provide a separate answer. Happily, the OP was unable to edit the answer, but was able to post two separate answers (under different usernames) trying, more or less, to disavow that answer. These answers have been both deleted by a(nother) moderator.
There is certain inconsistency here, I believe. Thinking hard, I do not know what would be correct actions in this situation. What do you think?
What puzzles me most is that the counteranswer might be found incorrect by somebody else who might indicate this in a separate answer. Then in turn the OP (or again somebody else) might find this separate answer incorrect and post still another answer explaining this, etc. etc., which would ultimately add to MO a piece of garbage.
PS Most probably there are some additional appropriate tags that I cannot discover. Please add them if you know some.