Even though sloppy is fine, it might be a bit of a good idea to arrange this particular page because, to be quite frank, as it is right now, it is just absolute garbage.
Doomed timelines are a phenomenon in Sburb based on the fact that many events are the result of closed time loops. A doomed timeline is created when a person fails to do what they are supposed to do causing the entire timeline and its inhabitants to be doomed. Interaction between doomed timelines and other universes is prevented; however a person can travel back in time from the doomed timeline and avert the timeline from ever being created. In fact this is at times necessary to succeed, meaning a doomed timeline has to exist in order to ensure its own non-existence.
It is not entirely clear how significant the change from the alpha timeline must be to create a doomed timeline and if paradox space only cares about outcomes or also the specific actions leading up to them.
It is possible to access memories from doomed timelines. The Rose Lalonde from Davesprite's timeline accomplished this by going to sleep before the doomed timeline is erased. According to Dave, dream selves operate somewhat outside the normal time continuum, allowing memories from alternate timelines to be passed on. It should be noted, however, that the only player to ever do this (Rose) is a Seer. She has said, that her alternate timeline memories come to her like visions. So it is unclear how much of this phenomenon might relate to her class.
Terezi Pyrope was able to access
the memories of her Game Over counterpart, along with the memories of other ghosts, using her powers as a Seer of Mind.
Squared Sprites are naturally able to access the memories of many of the alternate selves of their prototyped components.
Players that die in doomed timelines can appear in dream bubbles within the Furthest Ring. Doomed players who enter the alpha timeline also appear in dream bubbles after they die. Players that are still alive when a doomed timeline is erased do not seem to enter dream bubbles, as they cease to exist.
Aranea Serket is convinced that a doomed timeline is able to eventually take over as the new alpha timeline if it is "healed" by someone with extreme healing powers and luck. This is little more than her own speculation though.
It was once believed that the mortal consequences of travelling back from a doomed timeline could be evaded through one prototyping themselves, as shown by Davesprite, who managed to live three years after returning to the alpha timeline, journeying across the Yellow Yard. With the assistance of somebody who has become unstuck from reality and can thus alter the alpha timeline without causing offshoots, it is possible to leave a doomed timeline without being marked for imminent death. The instances of John, Roxy Lalonde, the surviving B1 sprites, and the residents of LOWAS native to the timeline shown in GAME OVER have managed to procure such a fate.
Humans' session
Three doomed timelines have been shown in the kids' session. However, there were likely others due to Dave's remarks about dead copies of himself. It is also possible that if that timeline did not affect the alpha timeline, none of the alpha players would be aware of its existence, and it would therefore have no bearing on the story. Of the three known, two were directly caused by Terezi. This stands to reason, as she is a Seer of Mind, and her role is to explore various possible outcomes.
The first brief one occurred when the Draconian Dignitary didn't receive any resistance to the theft of Rose's journals. Dave then traveled back in time to prevent the theft. DD killed that Dave, and seeing his body warned the alpha Dave not to repeat the mistake.
The second and most important one was created when John, under the guidance of Terezi, prematurely met Typheus and was killed by him. This indirectly affected the timeline in numerous ways. For example, John would no longer be able to perform the ectobiology needed to create himself, his friends and their guardians or function as Jade's server player. This would, in turn, prevent Becquerel's prototyping and prevent Jack Noir from becoming a threat to the trolls which would lead to them not trolling the kids thus not tricking John into prematurely meeting with Typheus. Alternate Future Dave originated from this timeline and prototyped himself, believing that the kernelsprite's preference for dead or doomed things would save him from death. Additionally, the Rose of this timeline was able to transfer her memories to her alpha timeline self, causing the awakening of her dream self.
The third doomed timeline was also created under the guidance of Terezi. She flipped a coin and made Dave decide which side of the coin would lead to him learning how to get to god tier immediately, and which side would delay the revelation. The flip itself was a constant across all timelines, but two alternate timelines were created by Dave's two possible choices. The Dave who chose "immediately" fell asleep on his Quest Bed, while the other, who chose "later" was eventually sent back in time to kill him. However, he was unable to go through with it, even though he knew that self was not from the alpha timeline, and was therefore doomed. Shortly after waking up and leaving his quest bed, the doomed Dave was killed by Jack Noir.
Post-scratch humans' session
Only one doomed timeline has been shown in the post-scratch humans' session, introduced in GAME OVER. It was stated to be a doomed timeline by Jade in a conversation with Calliope while referring to John's forced mastery of his retconning powers. This timeline was suggested to have been created by Aranea resurrecting herself with the Ring of Life and using Gamzee as a means to transport herself to the session and take it over. Every player except John, Roxy and possibly Dirk were either killed or mortally wounded in the ensuing bloodbath, which also destroyed the majority of the session's lands.
After gaining the ability to fully control his powers, John transported himself, the pre-scratch kids' sprites, LOWAS and its residents out of the timeline, and were shortly joined by their fellow instance of Roxy. The timeline was eventually averted with John transporting back to the Prospitian Battleship to retrieve the Ring of Life before Gamzee, the first action in a list of objectives given to him by Terezi in order to influence the previous sessions to have marginally better outcomes than before, and fully erased from existence upon the completion of the rest of these tasks and the placement of LOWAS into the new timeline's session.
Trolls' session
The trolls' session was plagued with at least a thousand doomed timelines, many of which Karkat blames himself for. However the army of Aradiabots produced by these ended up being critical in the fight against the Black King.
Interestingly, part of the genetic code found in first guardians that was needed to create Doc Scratch came from a doomed timeline. In this doomed timeline, created when Karkat did not explode his computer by running the Mobius Double Reacharound Virus, Gamzee killed all of the trolls and wrote his part of the code in Karkat's ~ATH manual, which was destroyed in the explosion in the alpha timeline. The Aradia from this doomed timeline took the book back with her as evidence and also ensured the explosion would take place by returning to Alternia via a reckoning gate. The alternate timeline version of the book was later stolen by Agents of Derse to create Doc Scratch.
Another doomed timeline seen was the result of Terezi being unable to bring herself to kill Vriska. In this timeline, Terezi allowed Vriska to leave the troll's meteor to fight Jack Noir. However, Vriska left a scent trail from her wings, allowing Jack to find the meteor and kill the other trolls. When he had accomplished this, he returned to fight Vriska. The outcome of their battle is unknown, though Doc Scratch has stated he wouldn't bet against Vriska winning (were he a gambling man). This timeline was envisioned using Seer powers and Cue Ball-granted omniscience, and so may have only been a potential timeline as opposed to an actual doomed timeline. Aranea implied that this may not be the case, however, as she knows about it and that Terezi caused it but is not a Seer.
While traveling through The Furthest Ring, the characters on the meteor encounter other trolls that died in doomed timelines. Some of these trolls had attained god tier before they died, including an alternate Feferi, an alternate Eridan, an alternate Tavros, and an alternate Karkat.
Pre-scratch trolls' session
Despite the pre-scratch troll session having a glitch that rendered the null session, leading to them scratching their session, the session, like its post-scratch counterpart, had a large number of doomed timelines. Alternate Damaras from these timelines were hinted to have sabotaged the alpha timeline, suggesting that the doomed timelines may have had some degree of success. However Damara's sabotage was hinted to not be an attempt to preserve the alpha but rather out of spite due to the actions of Rufioh, Horuss and Meenah.
Cherubs' session
Only one doomed timeline has been referred to so far in regards to the cherubs. In this timeline, Calliope's abstination from communicating and bonding with the kids allowed her to predominate over Caliborn. This instance of Calliope reached the god tiers but ended up dying eventually, most likely due to the Muse's version of The Choice involving sacrificing themselves to stop an uncontrollable tyrant/Lord and become a martyr that would benefit all who lived.