Whether a TV show follows one chronological timeline or attempts to interweave several time periods, it is still rather easy for it to plant a few mistakes here and there. Timeline plot holes are among the most common inconsistencies in series because they are difficult to catch in the grand scheme of things. Depending on the show, continuity errors can be irrelevant character details or significant plot holes that impact the show's comprehensibility. For instance, Friends notoriously got the characters' ages wrong multiple times, and yet it didn't hurt the popularity of the show at all.
In other, more severe cases, timeline inconsistencies can affect the series' general quality and, therefore, hurt its appeal. Riverdale's alternate universes and timelines in the later seasons turned away a great proportion of audiences from their respective screens solely because of how little sense the storylines made, compared to the events in the earlier seasons. Apart from Friends and Riverdale, quite a few shows made a mess of their own timelines, but some managed to either fix it to the best of their ability or render it so trivial that it was soon forgotten.
The Witcher
The Witcher season 1's multiple timelines are a mess
From episode 1, The Witcher hasn't even tried to make following the plot an easy task. Season 1 was infamous for how badly it handled its multiple timelines. None of the events or connections made any sense until the season 1 finale, but even then, a lot of things were left unexplained. In season 2, the series started focusing on a single timeline, but it was still too convoluted. In season 3, The Witcher even admitted to its confusing timeline on-screen when Triss Merigold mentioned, "It's hard to believe so much has happened in a week."
Doctor Who
Time travel with the Doctor is "timey whimey"
Since Doctor Who is a show about time travel, it is bound to have a few timeline plot holes. The series has retconned its timeline plenty of times, which is mostly evident from all the Dalek invasions that no one seems to remember once they are over. In addition, the Doctor does a lot of time traveling, and they often end up changing the past by accident with little to no repercussions to the present. The Time Lord's age doesn't add up either — there have been instances when the Doctor's later incarnation claimed to be younger than their earlier one.
Friends
The friends' ages & birthdays keep changing
Over the course of 10 seasons, Friends changed the birthdays and ages of the main six plenty of times. For instance, in season 7, episode 14, "The One Where They All Turn Thirty," Rachel celebrated her birthday in February, and later that season, she told a police officer that she was an Aquarius. However, in season 4, episode 5, "The One with Joey's New Girlfriend," Rachel informed Gunther that she was born on May 5, which would make her a Taurus. Plus, Rachel was 28 years old for three seasons and then 29 for two more.
How I Met Your Mother
Barney can't get a hold of his driving skills
How I Met Your Mother expertly shifted between the timelines, spicing up the present with the flashbacks and flashforwards, but a few continuity errors crept in regardless. Season 2, episode 17, "Arrivederci Fierro," revealed that Barney was afraid of driving. However, the character was confident behind the wheel soon after and even before the episode in question, like the time Barney drove Ted's mother to the airport. Apart from Barney's driving skills, Robin's history with sports and Marshall's experience with fighting stood out as obvious issues with the timeline of the show.
Brooklyn Nine-Nine
Jake & the Pontiac Bandit's history is confusing
Jake Peralta and Doug Judy were easily one of the fan-favorite duos in Brooklyn Nine-Nine, and the Pontiac Bandit became a vital recurring character on the show. In season 1, episode 11, "Christmas," Jake was jealous of Holt because the latter had enemies with cool names, implying that he had never had one. In the very next episode, "Pontiac Bandit," it was established that Peralta had been after the Pontiac Bandit for nearly a decade — and "Pontiac Bandit" is certainly a cool name. Holt's stance on food seemed to change over the course of Brooklyn Nine-Nine as well.
Riverdale
Riverdale's multiverse receives no proper explanation
Riverdale started out as a murder mystery and got progressively weirder and less believable over the course of the next six seasons. However, it was season 6 when the series stopped making sense altogether — five years of character development and world building went up in smoke, and the characters were transported into the timeline of Rivervale. Season 7 made it even worse since the show was suddenly set in the 1950s. From beginning to end, Riverdale was a wild ride that lacked coherent storylines and some kind of explanation for its plot twists.
Full House
Uncle Jesse can't remember his own high school years
Like any other sitcom, Full House's timeline wasn't always consistent, and Jesse's high school story arc proved that. In season 4, episode 9, "One Last Kiss," Jesse went to his high school reunion, and in the flashback, a younger Jesse was shown to be graduating. However, in season 6, episode 6, "Educating Jesse," Jesse admitted to being a high school dropout, so he went back to get a diploma. Jesse's retconned high school graduation is one of Full House's biggest plot holes. Danny's hygiene standards, which changed drastically by season 2, didn't make much sense either.
Supernatural
The Winchesters' grandfather remains a mystery
Supernatural ran for 15 years, so naturally, its world building evolved, causing ripples in the timeline. In season 4, episode 3, "In the Beginning," Castiel sent Dean to 1973 where he met his parents. At one point in the diner, a man came up to John and asked him to say hi to his "old man" for him, meaning that Henry Winchester was alive in 1973. In season 8, episode 12, "As Time Goes By," Henry time traveled from 1958 and died in the present time, so there was no way that he would be alive in 1973.
Legacies
Josie & Lizzie's ages aren't consistent
Being the third show in The Vampire Diaries' universe, Legacies had the hardest time following the timeline that was previously established. In The Originals, Hope was about two years older than Lizzie and Josie, but in Legacies, all three girls seemed to be of the same age — they attended the same classes at the Salvatore School. In addition to that, Legacies showed that the residents of Mystic Falls conveniently forgot that Alaric died a few years back, during The Vampire Diaries season 3, which was evidenced by his decision to waltz back into the local high school as the principal.
That '70s Show
The 1970s last a while for Eric & his friends
That '70s Show's wacky timeline could be attributed to the fact that the gang spent too much time in the circle, but it was still baffling. Even though the series lasted for eight seasons in real life, only four years passed for the characters — season 1 began in 1976, and season 8 ended in 1980. Yet, Eric and his friends managed to celebrate Christmas five times over the course of four years. The timeline errors didn't end there, as Donna's sisters also mysteriously vanished after the pilot episode and were never mentioned again.
