To quote our rules,
If you're not a moderator, don't act like one. If you see trouble, take ten seconds to report it and just move on with your life. This ranges from calling out someone else for violating the rules to telling people that their thread is a duplicate. If you take matters into your own hands, the problem will still be there, at best; at worst, you'll exacerbate the issue. The moderation will deal with it - that's their job, not yours.
If that isn't clear enough, consider this quotation, courtesy of Swoop,
If you could say the exact same thing to a moderator privately in a report, then don't go and post it publicly in a thread.
Why do we concern ourselves with backseat moderation? Put simply, posts that are made primarily with the intention of telling another user that they've done something incorrectly are completely off-topic and do nothing to further the discussion at hand - they ultimately serve no purpose other than to potentially derail a thread. Even if your intentions are good, people sometimes react in a hostile manner and the situation becomes worse. Let the mods handle things.
While we do also appreciate efforts to point somebody to the proper location when they misplace a post or thread, please report these instances rather than replying to them as members are unable to remove their own posts or relocate them. By filing a report, one of the moderators can facilitate this and instruct them in where things belong.