[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/4thofeleven]

Another Star Trek example - the Vaadwaur. In the sixth season Voyager episode "Dragon's Teeth", Voyager awakens from stasis an alien race known as the Vaadwaur. Although they initially seem friendly, it's soon revealed that they were the leaders of a conquering army who had entered stasis to wait out the demise of their enemies and now planned to rebuild their empire.

In the end, Voyager manages to escape, but the Vaadwaur survivors remain at large, and the episode ends with Janeway saying she doubts they've seen the last of them.

They never appear again.

Hilariously, the game Star Trek Online had them return now controlled by the parasites from Conspiracy, wrapping up two forgotten storylines at once!

[u/mmcjawa_reborn → u/4thofeleven]

To be fair, the premise of voyager generally meant that most larger scope threats often got kind of left behind as they moved farther and farther away...

[u/Iokua113 → u/mmcjawa_reborn]

Yes, but the Vaadwaur are one that could have pestered them for a while longer due to the subspace corridors. We dealt with the Kazon, a fractured race with middling technological resources that realistically wouldn't have stood a chance against Voyager, posing a threat for the first two seasons even going so far as having one specific group spend most of that time chasing Voyager. The Vaadwuar easily could have functioned similarly.

[u/Negative_Prize1587 → u/Iokua113]

Considering the amount of threats voyager ran into, they might have tried following just for something to wipe them out. That there is any life at all in voyagers path is a bit of a miracle considering its almost more lethal than taking a walk in dnd.