I'm more interested in the ones that get overlooked, and that are, in hindsight, even more devastating to the fictional universe than the examples listed.
Mine is from the TNG episode Emergence. The one where the Ship comes to life, and with the Train headed to New Vertiform City.
I'll quote from Memory Alpha
What this means is that at any time, in any place, without any warning, a starfleet ship can just explode.Later on, La Forge rounds up his investigation with Data on the bridge, finding only more mysteries. It shows that a theta flux distortion had been building up around the ship. The sensors were never designed to detect such distortions, yet there was a record of the distortion in the sensor log. Data reports that "one fact is clear however. The distortion was growing in strength. If we had remained at our original position 1.7 seconds longer, the distortion would have ruptured our warp core." La Forge tells Picard that if the ship had not entered warp when it did, the Enterprise would have been "blown to pieces."
Just suddenly, without warning or seeming reason, go boom.
This episode is very near the end of the run of TNG.
Lets instead go to the middle. Season 4. Now due to run times, commercial breaks, extra or fewer episodes, finding the exact episode in the "middle" of TNG would be hard to do. I'm going to pluck out Data's Day as my example as it is close enough to the dead middle of TNG for my purposes.
I'm going to use the script of that episode but make one minor edit.
I think you can see my point.CRUSHER: What?
DATA: I would like to learn how to dance.
CRUSHER: Why me?
DATA: It was in your service record. Awarded first prize tap and jazz competition, Saint Louis Academy.
CRUSHER: Okay, okay.
DATA: Have I said something to upset you?
CRUSHER: It's just that, that was a long time ago, and I don't want to be known as the dancing doctor. Again.
DATA: Then your answer is no.
CRUSHER: All right. But let's keep this between
SUDDENLY THE ENTERPRISE EXPLODES AND STAR TREK IS OVER NOW.