8. Star Trek: The Next Generation, "Conspiracy" - Starfleet Gets Infiltrated, And Then No One Mentions It Again
The Next Generation flirted with serialization several times throughout it's run (mostly in two -parters, and one excellent one-off in "Family") but it never really took off. there is no more painful example than "Conspiracy", near the end of season one. TNG foreshadowed this episode in "Coming of Age", and it all looked like there was going to be a big build-up to a season finale extravaganza of corrupt admirals and evil plots. what we got was... something else. It's not a bad episode by any means, but what is frustrating is how nothing that happens ever comes up again except in passing. The episode even ends with one of those, "They got off one final signal, so they know where we are and could come for us again" endings. This may not seem that controversial, but when you have a series as canon-obsessed as Star Trek, there's something a little off about this never being revisited.