There's actually some value in keeping wrong *answers* around, as long as they're *visibly wrong*. But comments are already a bit of a distraction - keeping worthless or outright *harmful* ones tacked onto a post is a luxury. So you have a few options: 1. reply, noting the error (so the author can delete it or correct it). 2. flag it, noting the error (so a moderator can delete it, or the system itself if enough folks flag) 3. both 1 and 2 Note that up-votes counter flags to a certain degree (at least when it comes to automatic deletion). See also: http://meta.stackoverflow.com/questions/17364/how-do-comment-voting-and-flagging-work