If someone posts an answer that is mathematically incorrect, should we flag it to delete, or just downvote and comment?

See this answer as an example.

  • $\begingroup$ tl;dr downvote and possibly comment if someone hasn't already. $\endgroup$ – Najib Idrissi Oct 15 '15 at 15:00
  • $\begingroup$ I see. I searched for "incorrect" before posting $\endgroup$ – kleineg Oct 15 '15 at 15:03
  • $\begingroup$ If the wrong answer contains an interesting mistake, it would probably even be more valuable to diagnose it in the comments, downvote/abstain and leave it. Someone might make the same mistake in reasoning later, and that would clarify it. $\endgroup$ – rschwieb Oct 15 '15 at 16:59

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