Television

Heroes.

The first half of the first season was so good. The second half was ok. Every subsequent season gave the impression that it was handed off to a different writer every week, and that those writers hated each other.

They changed the rules constantly, except when they just ignored the rules.

The main character collects new powers as the show goes on, but he never uses them. He just completely forgets that he has them. Kind of like the time he completely forgets he has a girlfriend and never mourns her, mentions her, or even acknowledges that he lost her somewhere in an alternate timeline or something.

That's not much better than the villain. The entire first season is about stopping him from doing a specific thing, because a time traveler has foreseen that it'll result in the world ending. Season two opens with him doing that thing, and everything is fine. He has at least three heel-face turns, which are immediately undone when the next writer is up to bat.

One good character undergoes a terrible transformation and murders a bunch of people, but the next season he's suddenly good and everyone forgot. Again.

Constant retcons. This character is actually that character's secret brother! This one lady who died was actually triplets! It adds nothing to the story and makes no sense, but there it is!

(I'm not joking either. Secret triplets.)

They did a reboot of the show a few years later and did the exact same thing.

Homer Simpson has forgotten he knows music at least 2 times.

Other than that, I’ve heard Lost is bad, very bad.

Far from the worst but my funniest example comes from my favourite show. In the first season of Stargate SG-1, they introduced an alien weapon which would go on to become a staple of the series, the Zat Gun. One shot stuns, two shots kill, and three shots... Disintegrates?!

Yeah, it's so stupidly powerful that the writers pretty much immediately realized the mistake and the show kinda just conveniently forgets that was established in the first season, except for like three other times in the entire ten season run.

Even the first two shots became very nebulous over time.

Edited to add, the disintegration effect is even mocked in-world in a very meta way in later seasons.

If I’m being honest with myself, Star Trek.

All of it.

Baywatch, especially when you take into account Baywatch: Nights where Hasslehoff works as a private eye and solves x-files.

Ok but keep in mind I'm speaking to the televised order of release.

Firefly.

They released the series out of order so the very first episode already had you deep into the story without establishing anything or anyone. Once you streamed it or watched it on dvd it made more sense. Plus the choose not to air some episodes at all which also introduced plot points and by not airing those you ruin consistency.

And I'll add, the series is fantastic and serenity was a very nice bow to the short lived series. Highly recommend if you haven't seen it.

I won't say it's the worst, but King of the Hill had a habit of contracting itself. Especially when it came to Peggy's family.

Supernatural eventually just made it a running gag that things changed when they felt like it.

Days of Our Lives

Transformers. Is "energon" a crystal, a liquid, or something more nebulous? Are Primus and Unicron one being? Did the Quintessons create the Transformers? Where do the Go-Bots fit in?

And don't get me started on the Allspark vs. Vector Sigma...

Rings of Power sure is up there.

Futurama freely makes shit up. They'll invent new math theorems to make a body-swap episode work, but if their cool new backstory for Bender contradicts canon, they do not give a dang.

It's not the worst plot hole, because it doesn't affect the extremely episodic show. They're just willfully apathetic toward consistency.

1899 on Netflix.

Every episode had some big in-universe mechanic reveal which made pretty much every action taken by characters before that even MORE nonsensical.

It just kept getting worse and worse as events cascades into each other.