From sci-fi spectaculars to sitcoms, it's not unheard-of for our favourite TV shows to go back in time – so to speak – and rewrite a major plot point from series past.
Sometimes, it's a purposeful rewriting of the show's mythology for shock value. More often, though, this use of "retroactive continuity" – retcon for short – is to account for a sloppy inconsistency, or an error on the part of the writers.
But fans pay attention, y'see, and we'd always notice these glaring rewrites, even in the days before Wikipedia and fan sites preserved every last plot detail for posterity.
Here are some of the biggest TV retcons going, starting with a show that's no stranger to changing history.
1. Star Trek - The Klingons' appearance
The look of Trek's most iconic alien race has altered several times since their debut in a 1967 episode of the original series, 'Errand of Mercy'. Starting out as something close to human, they later adopted a more bestial look with ridged foreheads and snaggled teeth.
The real-world explanation was, of course, simply an advance in make-up effects, but the discrepancy bugged fans for years until a pair of Enterprise episodes in 2005 explained away the old Klingon look (sans ridges) as the after-effect of a virus.
2. Friends - Chandler knew Rachel all along
It's pretty evident from the Friends pilot episode that none of the Central Perk gang (bar Monica and Ross) are familiar with Rachel when she first bursts into the coffee house, having run out on her wedding.
But later episodes established that Chandler had met Rachel on several occasions prior, going all the way back to her high school days. They even made out once at a college party.
It all contradicts their 'first' meeting in the pilot, as well as scenes in season two's 'The One with the Prom Video' where the gang – including Chandler – are shocked at Rachel's pre-nose job appearance. The writers never bother to explain the inconsistency.
You can go here for more inconsistencies from Friends, including Joey's erratic sleeping habits and Ross's wavering attitude towards ice cream.
3. Red Dwarf - Lister and Kochanski's relationship
The first two series of Red Dwarf establish that Lister lusted after Kristine Kochanski from afar, never building up the courage to ask her out. The show's third episode, 'Balance of Power', even establishes that the pair only ever shared a total of 173 words.
But from series four's 'DNA' onwards, the nature of their relationship changes... in the sense that they actually had one, before Kochanski's tragic demise. The retcon has Kochanski breaking up with Lister after a short romance, a change made, according to writer Doug Naylor, because it felt "unrealistic" to have Lister still be pining for a woman he'd barely known.
4. Smallville - Jimmy Olsen isn't the real Jimmy Olsen
In 2006, teen Superman series Smallville cast Aaron Ashmore as Clark Kent's pal Jimmy Olsen – and make no mistake, this was the real Jimmy, with Ashmore saying it felt "amazing" to play the iconic character.
Except, the show later killed off 'Jimmy', revealing after three years that the character played by Ashmore was in fact Henry James Olsen, older brother of the established comicbook character.
Smallville never bothered to explain why Henry would have adopted the nickname Jimmy when that was literally his little brother's name – and, to confuse matters further, Ashmore also played the real Jimmy, all grown up, in a flashforward scene in the show's finale. Gah.
5. Heroes - Sylar ain't bad after all
A classic example of a villain so popular, the writers later retconned their evil actions to make them seem that little bit less dastardly.
Rather than have Sylar – the show's one-time Big Bad – continue to exist as an utter bastard who loves to kill, torture and maim, Heroes explained away his bloodlust as being driven by a terrible "hunger", a side-effect of his special power of intuitive aptitude.
Well... we guess that's all right, then? He couldn't help it, see? Even if he did seem to be rather enjoying all the murdering at the time. (See also the lesser sequels to John Carpenter's 1978 horror classic Halloween, which established that homicidal monster Michael Myers was compelled by a magical curse to slaughter his entire family. Riiight.)
6. Buffy the Vampire Slayer - Spike's sire
In his first appearance on Buffy, season two's 'School Hard', bad-ass bloodsucker Spike refers to former ally Angel as his "sire", a term commonly understood in the Whedonverse to refer to the vampire who 'turned' you. A vamp daddy, of sorts.
Yet later episodes establish that, while Angel was Spike's mentor, it was actually Drusilla who "made [him] a vampire" – so what's with the "sire" chat?
Joss Whedon later went on record to explain that sire can actually refer to any predecessor of a vampire's life – so Angel was Spike's grand-sire, if you like. It beats "the writers messed up" as an explanation, we guess.
7. Prison Break - No one is ever actually dead
The character of Sara Tancredi was definitively killed off when actress Sarah Wayne Callies couldn't agree a deal to return to Prison Break for its third season.
Tancredi was first kidnapped and later executed. The bad guys even delivered her decapitated head to our heroes as a warning. Pretty final, right?
Nope – Callies later struck a new deal and Sara improbably returned to the show. The head? It belonged to someone else. Don't worry about it. Wouldn't someone have noticed the difference? No, the thing was all icky, they didn't look too closely.
Still, at least Prison Break never stretched credibility quite that far again, with the later demise of series lead Michael Scofield sticking for good.
