A retcon can mean a few things. Sometimes retcons are genuine "gotcha!" moments from a team of smart writers, subverting audience expectations to pull off a major twist or reveal. Or sometimes they're genuinely interesting new pieces of information that complicate or deepen the lore of your favourite TV world.
But sometimes they're writing decisions that genuinely make the audience scratch their heads. Whether they appear to be new pieces of information that just don't fit right with what we knew before or are just genuinely horrible twists that make no sense within the context of the show, retcons can often feel like lazy afterthoughts that writers introduce for cheap new conflict, to chicken out of the stakes they'd set for themselves, or to desperately wrap up a loose end with out-of-character decisions.
Writing long running TV shows can get muddled, and often new pieces of information make for really interesting new plotlines to keep the show fresh. But sometimes they're executed so poorly that they end up illogical or lazy retcons instead, and in the process piss of their legions of loyal fans that expect better.
10. Roseanne - It Was All A... Book?
This one has three retcons for the price of one; two that were extremely controversial and almost came across as trolling, and one that fixed it decades later.
This one gets a little hard to follow, but the general gist is that season 9 of the classic sitcom Roseanne revealed that almost all of the events seen in the show up until that point were fictional versions of the truth, as the character of Roseanne is seen as an aspiring author writing half-truths about her own life.
So the first retcon was the spin on "it was all a dream", which never goes down well, as it just leads audiences into thinking that they've wasted their time. The second retcon was that Roseanne's husband - Dan - had actually died at the end of season 8, and all of his involvement in the semi-fictional season 9 was Roseanne dealing with her grief. This essentially made the entirety of season 9 - if not the whole show - worthless, as it all turned out to be false by the final episode.
The third retcon is a positive one that accounts for the sheer laziness of the "it was all a dream/book" retcon from the original show, with the Roseanne revival airing in 2019 - 22 years after season 9 finished. The revival reveals that the heartbreaking, but still lazy, retcon of Dan's actual death was - you guessed it - the fictional ending of Roseanne's book. He's still alive, and the retcon that angered every Roseanne watcher in the 90s was all a ruse. It just took 20 years and three retcons to get to it.
For what it's worth, the revival's rewriting is just as lazy. But at least it fixes something that pissed everyone off.