Derailing/Playing With

Basic Trope: Forcibly changing a subject under discussion (or a group's goals or plans) away from one topic or objective either against the will of the participants or by persuasion, toward one desired by the person changing it.
 * Played Straight:
 * In a discussion among feminist writers who see the Victim Falls For Rapist trope as offensive about how to deconstruct it, Bob joins the discussion and begins talking about freedom of speech and how men have the right to write whatever they wish.
 * A Troll posts to a discussion about Police Brutality saying the victims deserved it (with optional racial slurs) and going on at length over how difficult police officers' work is.
 * Exaggerated:
 * Bob, unable to win any points with his arguments, turns into the Garbage Post Kid and Spammer, finally causing that discussion and any other discussion like it to be shut down simply because of his Nausea Fuel and Triggering images until he gets banned.
 * The derailed discussion becomes instant Internet Backdraft, with all of the parties involved now screaming at each other and starting other fights outside of it rather than recognizing what had happened.
 * Inverted: A reasonable person comes to a venue infested with Trolls, SockPuppets, MeatPuppets, and all types of The Shill to discuss a controversial issue, staying on topic and refusing to get distracted into a Flame War. The residents are so shocked with the on topic and intelligent commentary that it stays relatively peaceful.
 * Justified:
 * The original discussion is a Troll posting or incipient Internet Backdraft.
 * The original discussion has reached its logical end and the derailing is more due to the Left Fielder showing up and getting bored.
 * The topic that was derailed onto was actually an important topic in and of itself that developed out of the original discussion.
 * Subverted:
 * Bob's derailing attempt and his postings to the discussion only served to show just how creepy some fans of Victim Falls For Rapist who go out of their way to support it can be.
 * The angry, racist pro-police Troll in the Police Brutality discussion gives people a far more negative opinion of the police than they had before he showed up to defend them.
 * Double Subverted:
 * One of the feminist writers in the discussion finds Bob's controlling, domineering behavior sexy.
 * The Troll in the Police Brutality discussion is sufficiently a scary enough Internet Tough Guy to silence the discussion.
 * Deconstructed:
 * Bob is shown to be a stereotypical Otaku virgin, incapable of even leaving his room. His derailing attempts are shown to be the result of his poor relationship with his mother and his suffering a severe case of GIFT. Once he gets his Shut UP Hannibal from the writers he attacked, he realizes just how pathetic he is and gets psychological help.
 * Someone investigates the racist pro-brutality Troll and finds out that yes, he is actually a police officer instead of being the armchair warrior people suspected. After an Internet Counterattack showing exactly what he thinks of the people he's allegedly "protecting and serving," he is told to Turn in Your Badge and put under investigation by internal affairs.
 * Reconstructed:
 * Despite Bob's issues, he did actually have some very good points about freedom of speech and freedom of the press - he just expressed them very badly and in the wrong venue. In a later discussion about legally banning the new manga "Bastard Boyfriend" over its scene with the titular character engaging in Victim Falls For Rapist, he makes a very cogent argument over why the manga should not be banned by law.
 * The Troll posting to the Police Brutality discussion just happened to be reflecting the thoughts of his entire police department. The Internet Counterattack this time takes it to the national media, and some people end up actually thanking the Troll because without his abusive, angry, racist postings, no one would have known about the massive case of Bad Cop Incompetent Cop for which he became the unwitting spokesman.
 * Lampshaded:
 * "Was there even a topic to be on here?"
 * "You're derailing. Please stop."
 * Parodied: The "This thread is now about trains" example from the main page.
 * Averted: Comment moderation keeps the discussion on topic.
 * Enforced:
 * "No topic" discussions, spam forums, a certain Image Board, and other places where off topic posting is encouraged.
 * As a way of dealing with Trolling in some places.
 * For PR professionals and sometimes as advice in dealing with Internet Counterattack.
 * Invoked: "I think Bob is derailing this discussion."
 * Defied:
 * When Bob arrives in the discussion with his first post, it is called out as derailing and he is banned.
 * The Police Brutality discussion had, in the first posting, a rule that anyone outright defending police brutality or using racial slurs would have their posts deleted and they would be banned. When the Troll posted, his post was reported and this happened. The discussion went on relatively peacefully afterward and nothing of value was lost.
 * Discussed: "If we ever have a thread on Controversial Topic X, let's specify what behavior we expect of the posters and post a link to Derailing for Dummies."
 * Conversed: "It looks like we have a Troll in here trying to derail this thread. What should we do?"
 * Played For Laughs:
 * Played For Drama: