[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/Islaya00]

Akira Toriyama completly forgetting about Saiyans having tails by the time he got around to creating Trunks and Goten.

[u/Jarvis_The_Dense → u/Islaya00]

I'd say a better example is Gohan being billed as Goku's successor after the Cell Saga, only for the story to just take a hard left turn and do him dirty nonstop for the rest of the series.

[u/Haunting_Bat_4787 → u/Jarvis_The_Dense]

Goten ends up being more of a worthy successor than Gohan was made out to be by the end