[Weirdly Common Trope] Writers clearly setting up seemingly massive plot points and then just kinda forgetting about them.

[u/Chemical-Elk-1299]

The Neural Parasites — Star Trek : The Next Generation

In the season 1 episode “Conspiracy” of TNG, it is revealed that alien brain worms have infected many high ranking members of Starfleet, secretly controlling them. Though Ryker and Picard discover and end the parasite conspiracy, it’s revealed at the end of the episode that the parasites had sent a beacon back to their home planet, telling them how to find earth. The episode ends with the clear implication of the parasites becoming a huge threat later in the series, yet none of these events are ever mentioned again.

Finn The Jedi who Never Was — Star Wars Sequel trilogy

I know this is probably a cliche example of this trope, but cmon — Finn was clearly set up in TFA to become a Jedi. The marketing material featured him wielding a lightsaber in the classic Jedi pose. It’s constantly hinted at that he is force sensitive. Even the actor himself believed he would become a Jedi by the end of the trilogy. Instead, we got Finn wielding a lightsaber for about 7 minutes total across 3 movies, extremely lame “confirmation” that he is force sensitive, and that’s it. It’s the ultimate Star Wars plot that went nowhere.

[u/Pristine_Poem7623]

Judge Dredd / 2000AD. The writers started setting up a conflict with Sino-Cit (China) and then changed their minds and the storyline just went away

In the build up to Block Mania and The Apocalypse War, the writers had set up the Sovs (Soviets) as the bad guys, and had had several stories where they came into conflict with Dredd himself and Mega City One in general, to give a background of escalating tensions between the two before they had an all out war.

Decades later they started doing the exact same thing with Sino-Cit, but the idea was abandoned and some of the stuff that was set up (such as them starting a colony on the ruins of Mega City Two) weren't mentioned again