Here is a non-comprehensive list of the many, many, many (many)time loops in the series. Given the nature of the trope, all spoilers will be unmarked:
- Mikkel goes through the time portal, ends up in 1986, and grows up to be Jonas's father Michael. And Jonas, haunted by hallucinations of his dead father, inadvertently loses track of Mikkel in the woods, which is what leads to Mikkel going through the time portal in the first place. Adding to that, in Season 2, we learn that Mikkel was deliberately taken through the time portal by a slightly-older future version of Jonas, in order to maintain the loop and ensure his own existence.
- Tannhaus's machine is only completed in 1986 after the Stranger allows him to study a completed future version of it. And he only started it in 1953 because Claudia came from the future with the blueprint and asked him to build it.
- Jonas eventually becomes the Stranger. And he sets himself down the path to becoming the Stranger, by giving him a map explaining where the time portal is, giving him the letter explaining his father's true identity etc.
- According to Noah, the Stranger's attempt to destroy the black hole somehow leads to its creation. Possibly Noah is referring to the fact that it sends Jonas and Helge off on their respective journeys to manipulate the timeline.
- In Season 2 the aged Future!Claudia takes a number of steps to manipulate and influence her younger self into becoming the time-traveling mastermind that she is. She's the one who abducted Claudia's dog Gretchen from 1953 and brought it to 1986, to spark her younger self's curiosity about time-travel. She then tells her younger self about time-travel, gives her the time machine, and sends her protegee Jonas to guide her younger self after her death. Claudia even leads herself to her own death - arranging for her younger self to receive a copy of the newspaper article of her death so that she knows when and where she needs to be for it to happen.
- In Season 2, Jonas uses the advanced time machine of his oldest future self Adam to travel to 20 June 2019, in an attempt to prevent the suicide of his father Michael the following day, which he believes kickstarted the entire tangle of causal loops. However, not only does he fail to prevent the suicide, but it turns out that he's the one who gave Michael the idea to commit suicide in the first place, which Michael willingly does in order to preserve the chain of events that leads to his son's birth. Moreover, even Michael's suicide note is a product of a predestination paradox - he only writes it because Jonas shows him the future version of it. It turns out this was all part of Adam's plan.
- Egon's death in Season 2 is a particularly tragic example of this trope. Claudia, while visiting the future, learns that Egon is found mysteriously dead in his apartment on June 26 1987. Returning to 1987, she's determined to prevent her father's death, believing that he's murdered by someone connected to the time-travel conspiracy. She goes to his apartment, trying to get him to come live with her and give up investigating time-travel, and during an argument with him, accidentally pushes him and causes the very death she was trying desperately to prevent.
- Adam kills Martha in front of Jonas, which instills enough hatred in Jonas to work with Claudia in order to subvert Adam's machinations in the time loops, and hope to save Martha. He eventually grows into the Stranger and loses faith in Claudia after her multiple manipulations, guiding 2019!Jonas to his future while preparing to stay in June 2020 to prevent the Apocalypse and save Martha, disregarding his own advice to not interfere with the past. Predictably, his efforts to save Martha fail as she is thrust into the position where Adam kills her. After this catastrophic failure, Stranger!Jonas is thrust into June 1888 with Bartosz, Magnus and Franziska, as they couldn't return due to the Apparatus overclocking, as well as the Apocalypse devastating Winden and most of the world moments later in 2020. To survive in the past, the travelers eventually work with Tannhaus to create Sic Mundus, where Stranger!Jonas becomes obsessed with the Origin of the Knot to change back their destinies. Jonas's attempts to break the Knot led him to create the Tesla Coil Machine which disregards the 33-year cycle, but heavily disfigures Stranger!Jonas into Adam. After learning the hard way he couldn't do anything to change anything at all, Adam eventually gives up and waits for his young self to meet him, as he manipulates Jonas into causing his father's suicide and makes him watch as Adam kills Martha... eventually ensuring his own existence.
- In Season 3, it is revealed that the entire Nielsen line is the product of a Stable Time Loop. Jonas (the one who goes to Eva's World) and the alternate-Martha have a child. This nameless child (at-least by the show) fathers Tronte, who in turn fathers Ulrich, who in turn fathers Mikkel (who fathers Jonas) and Martha. This loop extends to the Nielsen line on both worlds, which respectively produce the versions of Jonas and Martha who conceive the nameless child.
- Season 2 revealed a case of My Own Grampa, wherein Charlotte and Elizabeth gave birth to each other and were each others mothers and daughters. Season 3 adds another layer to this loop, revealing that a time-traveling Charlotte and Elizabeth were the ones who kidnapped the infant Charlotte and took her back in time to be raised by HG Tannhaus...which is what ultimately leads to Elizabeth's birth, which ultimately leads to Charlotte's birth, and so on ad infinitum.
- Ulrich and Katharina bullying Regina in 1986, ironically, paved the way for the former's own existence, as Boris, who will later become Aleksander intervening between them, saving Regina. It leads to Regina falling in love with him and marrying him. They will later become parents to Bartosz, who in 2020, due to interference from Ulrich and Katharina's grandson Jonas, will end up in 1888, meeting Silja, which is again due to Jonas (now Adam)'s interference. Bartosz and Silja will later become parents to Agnes and Noah, the former being mother to Tronte, who's the father of Ulrich, allowing him to exist to (unknowingly) torment his ancestor.
- Much like people, various things travel in an infinite loop - on of them is a pocket watch carrying the inscription "For Charlotte". We first encounter the watch in the hands of Young Elisabeth in Season 1: Noah gives it to her, telling her that it used to belong to her mother. The next time we see it is in Season 2, in Tannhaus' workshop in 1987 - he seems to work on it when Claudia arrives to have him explain the time machine. Elisabeth has the watch with her in 2020, when she enters the bunker before the apocalypse hits, and at first, the watch continues its way normally, staying with her until 2041. It is in the room with Baby Charlotte, when Adult Elisabeth and Adult Charlotte come to take her away on Adam's orders, and Charlotte grabs it, later leaving it with Tannhaus as the one keepsake from Charlotte's past. The path of the watch ends with Tannhaus gifting it to a sixteen-year-old Charlotte, but it started much earlier: In 1888, Tannhaus grandfather, Gustav, keeps it as a memory of his mother, also called Charlotte, who in turn received it from her husband Heinrich. After the Unknown kill him to keep him from revealing the secret of the time travelers to the world, they take the watch and bring it to Eve. Adult Martha later leaves it in Adult Jonas' room with the letter Martha wrote to him; he will give both to Noah at different points later.
- Alt!Martha meets Jonas, Bartosz, Magnus and Franziska in 1888 after the four of them get stuck escaping the Apocalypse. She decides to gain Jonas's trust by handing him her last cesium, and later outs Jonas as Adam in front of them. Jonas begins to create a new God Particle using the cesium, which evidently worked as Adam will later show Jonas, and the stress of being discovered causes Jonas to find a way to erase their existences, leading him to be Adam. After killing Martha in 2020, Adam relocates Sic Mundus to 2053, in which he orders Alt!Martha to 'help' his younger self by giving him her cesium.
- Regina is killed by Tronte in post-apocalyptic Winden, after which Claudia delves into ways to save her daughter. After many years, Claudia kills her Alternate Self and gets the golden orb from her and uses it to travel between various points in time in both worlds. After realizing the truth about both worlds being offshoots of a third world where Time Travel was never discovered, Claudia brings Tronte to Regina's grave in the future and requests him to Mercy Kill Regina, so that she (her younger self) does everything she can to prevent said death.
- A minor loop involving Katharina's name in Season 3. At the end of Season 2, Hannah travels back to 1954 and adopts 'Katharina Nielsen' as her alias. She meets a young Helene Wolf, who takes a liking to the name Katharina, and later gives that name to her own daughter...who ends up being the Katharina that Hannah named herself after!
- Similarly, the name 'Charlotte Doppler' results from a loop. After Charlotte is seemed to have perished in the Apocalypse in 2020, Noah and Elisabeth decide to honor her by naming their baby after her. After the baby Charlotte is kidnapped by her adult version and her mother Elisabeth, she is raised by Tannhaus to maintain the family's existence. After growing up, Charlotte vanishes in the Apocalypse to 2053, leading (the younger) Elisabeth to think she's dead, naming her daughter after her 'mother'.
- In a similar way, the family name Nielsen exists only in a loop. When in 1954, Hannah uses the name "Katharina Nielsen", maybe to spite Katharina, whom she feels has taken everything Hannah should have had - including Ulrich. To ensure that this is possible, Agnes (whose actual name should be Agnes Tiedemann, due to her parents being Bartosz Tiedemann and Silja Tiedemann) uses the name "Nielsen" when coming to Winden one year earlier to drop off Tronte. Tronte of course uses the same name, marries Jana and has Ulrich with her, who later goes on to marry Katharina, making her Katharina Nielsen.
- In another way, the nameless son of Jonas and Alt!Martha takes on the surname Nielsen and meets Agnes and marries her, divorcing sometime after Tronte is born. Agnes keeps the name, and so does Tronte, starting the Nielsen line which eventually gives birth to Alt!Martha who goes by Nielsen and gives her surname to the nameless son.
- The Incident in Adam's and Eva's Worlds which created the wormhole also results from a bootstrap paradox. After Jonas (himself having made use of the wormhole in Adam's World) makes Alt!Martha aware of the wormhole in the Caves leading to various time points, Martha (as Eva) eventually decides to maintain the Stable Time Loop infinitely. She sends her son, The Unknown to steal the plans of the Winden Nuclear Power Plant from the director's office and the master key from Bernd Doppler, leading to the deaths of Bernd and the secretary, Jasmin Trewen. After reading the reports learning which area was sabotaged, Eva sends off The Unknown. The child and the old Unknown then trigger the Incident in their own world, whereas the adult one goes to Adam's World in June 21, 1986 to trigger the Incident there, leading to the formation of the wormhole in both worlds used by our protagonists (and several others in their own journey in the Stable Time Loop) and the barrels which will also eventually cause the Apocalypse in both worlds.
- After confirming that the boy in the morgue is Mads, Ulrich (from Eva's World) goes to Helge who confirms the truth, and mentions he needs to stop 'Ulrich' and is surprised he's alive. Confused, Ulrich lets him go, and follows him to 1986 (instead of 1953 in Adam's World). He meets the older Helge Doppler this time (now totally complicit in the kidnappings) and bludgeons him to give him that scar across his eyes. Old!Helge arrives and kills Ulrich to protect his younger self, which was witnessed by the younger one.
- Adam after failing to destroy the timelines by killing a pregnant Martha, goes to her world to kill the original Martha, meeting Eva. Adam then shoots her as she dies, after which her body is discovered by a younger Martha. After witnessing to what extent Adam can go to destroy the loop, she finally adopts the name 'Eva' and manipulates the Knot and everyone else further to stabilize it, eventually growing into the older Eva who will be killed by Adam. Though it doesn't happen again in the finale, meaning that the loop is broken.
