This trope exists to provide an In-Universe explanation on why time traveling is forbidden. Maybe is to avoid a Butterfly of Doom scenario, the creation of an Alternate Universe, or to avoid the pain in the ass to question how things worked out and you became your own grandfather. Maybe time itself has a way to punish those who mess with time, by sending the Clock Roaches, or worse, messing with time will unleash them to destroy all of existence!
Contrast Chekhov's Time Travel, where there is typically an In-Universe justification for the need to use it.
Anime and Manga
- Dragon Ball Super
- During both Dragon Ball Z: Resurrection 'F' and the arc that adapts it, Jaco explains to Bulma that time manipulation is illegal according to the Galactic Patrol. When Whis technically travels back in time by rewinding the clock three minutes, he turns a blind eye mostly because it is pointless for him to say anything. Likewise, both he and the Galaxy King don't bat an eye when Hit's time abilities are shown in the Champa saga, mostly because it is also pointless.
- After Future Trunks goes to the present to escape from Goku Black, who has been destroying his home in the future, Whis states his surprise that Earthlings managed to create a time machine, but also say that time traveling is a taboo to the gods for the Butterfly of Doom justification. Trunks was lucky Beerus was in a good mood and allowed the heroes to keep traveling in time to deal with the villain, who had a Time Ring. This trope is explored further with the Time Ring, a magical artifact that the Supreme Kais use to travel into the future to see how a civilization has evolved through time, and every time a new timeline is created by using a time machine, a new Time Ring of green color appears as a way to travel to that Alternate Universe. In fact, one of the reasons Goku Black hypocritically proclaims he invaded Trunks's timeline was due to him sinning, by traveling in the past the first time and saving Goku from his heart disease.
Goku Black: You've committed a sin far greater than me. Trunks, you're one of the reasons why I wished for the extinction of humanity, by rewriting the past, a new Time Ring was created. That itself is proof of your sin. How many times have you came back through time?Zamasu: We know. We know that you continued committing a taboo among gods, despite being a mortal.
- After the Future Trunks arc is over and Trunks returns to his own era, Bulma tries to recreate another time machine despite Beerus warning her it's forbidden. Without the presence of a menace like Goku Black to the timelines, he destroys the time machine once and for all, to Bulma's misfortune.
- Time travel is illegal or highly restricted in some incarnations of the Legion of Super-Heroes, most notably the post-Zero Hour Legion, and the DC Rebirth Legion. In both cases, Brainiac 5 assumes this doesn't apply to him.
- A similar principle is brought up in Doctor Strange (2016), where time-based sorcery is considered forbidden among the Masters of the Mystic Arts, with the Eye of Agamotto their most powerful relic used to house and control the green Infinity Stone (also called the Time Stone) being their most valuable relic. They explain that tampering with the space-time continuum is considered a violation of natural law, the Masters themselves existing to protect such laws.
- Star Trek:
- Star Trek: The Original Series: The franchise's Ur-Example of this trope involves the planet Gateway, from "The City on the Edge of Forever". The Federation is nearly erased when Dr. McCoy unwittingly changes history by saving a 1930s peace activist who delays the USA's entry into World War II, enabling Nazi Germany to win the conflict and Take Over the World. After this situation is resolved, the planet is placed under strict quarantine. Some non-canon licensed works up the ante to the same death penalty used for Talos IV.
- Star Trek: Voyager and Star Trek: Deep Space Nine: These two concurrent series simultaneouslynote introduced the timeline's current Department of Temporal Investigations and the future Temporal Prime Directive. The former is relatively flexible and informal, but it introduces a taboo to the series canon. The latter is enforced by 29th century officers who work to preserve the timeline, and will go so far as to eliminate people from history to do so.
- Star Trek: Enterprise goes so far as to depict a Temporal Cold War between factions trying to alter history for their own benefit. It also puts a Time Police officer aboard Enterprise until he gets killed...and comes back to give Archer instructions without explaining how he recovered from being dead.
- Star Trek: Discovery season 3 reveals that, in the 32nd Century, Time Travel is explicitly forbidden after the mess that was the Temporal Cold War. In fact, the Guardian of Forever itself left Gateway and went to another planet because it got tired of everyone using the gateway!
- In The Dresden Files, time travel is one of the acts forbidden by the laws of magic, with death as the penalty for a single transgression. The exact reason is not given, but presumably has to do with paradoxes or other problems.
- In the Doctor Who New Adventures, there are technological measures (the Backtime Buffers) preventing attempts to take a TARDIS into Gallifrey's past, which is strictly forbidden.
- In The Stainless Steel Rat Wants You! the protagonists are mulling over the idea of ending a war with a bunch of creepy crawly aliens by just sending all the aliens a hundred years forward in time (by which point humans will presumably have figured out how to deal with them) when an agent of the Time Police suddenly appears (right in the middle of their "secret meeting") to inform them that this solution is forbidden. An interesting variation in that the heroes aren't even planning on sending their enemies into the past—which could raise obvious paradoxes—but into the future, but it's still forbidden by the Time Police.
- In Crimestrikers, crime syndicate Outrage discovers Tempestium, a rare element that makes time travel possible. They use it to bring three historical villains, collectively known as the Time Terror Team, into the present (although a hero from the past is brought along accidentally), which causes the setting's benevolent One World Order to round up all the Tempestium they can find and lock it away before it can cause more trouble. So the world is safe — at least until more Tempestium is discovered...