<p>There's actually some value in keeping wrong <em>answers</em> around, as long as they're <em>visibly wrong</em>. </p>

<p>But comments are already a bit of a distraction - keeping worthless or outright <em>harmful</em> ones tacked onto a post is a luxury. </p>

<p>So you have a few options: </p>

<ol>
<li>reply, noting the error (so the author can delete it or correct it).</li>
<li>flag it, noting the error (so a moderator can delete it, or the system itself if enough folks flag)</li>
<li>both 1 and 2</li>
</ol>

<p>Note that up-votes counter flags to a certain degree (at least when it comes to automatic deletion). See also: <a href="http://meta.stackexchange.com/questions/17364/how-do-comment-voting-and-flagging-work">How do comment voting and flagging work?</a></p>