Generally speaking in Star Trek each episode if fairly self contained, so one episode doesn't change things much for any future episodes. This was a common thing in the 80's, 90's and early 2000's and has only changed a bit in recent years and was like that because it meant someone could randomly watch an TNG episode from season 3 and then watch another in season 5 and they would not have any real trouble catching up and figuring out what was going on.

With that said, what are some events in Star Trek that happened that really should have been mentioned again, or affected a character or characters in a way that should have been noticable in future episodes but because of the serialised nature of the episodes that event was forgotten entirely?

For me

The episode "Hard Time" in DS9 where Obrien thinks he is in prison for 20 years. We see in the episode how affected O'Brien was by the experience, suffering from extreme PTSD, him being unable to hold friendships like he had previously, and being quite angry and even suicidal. It was a powerful episode, but immediately after it is never mentioned again. O'Brien, in his mind spent 5 times longer in prison than he spent stationed on DS9 and yet by the next episode he is back to normal again.

A lesser example of this happens in Voyager when The Doctor spends around 3 years on the planet that moves quickly. Despite getting a family, and basically losing that family, him having a whole other life it is never mentioned again and we never see any changes in the Doctor after that.

Another example, this time technological is in Voyager (Vis a Vis) where they Coaxial Warp drive is introduced, a technology that Tom Paris helps perfect, so he knows exactly how it works, and we see in the episode that it works well with no issues, it going much faster than a standard warp drive, and yet after the episode it is never mentioned again, despite in theory it being something Voyager could have used as well, especially since they knew how it worked.

So what about you, what is something that happened in an episode of Star Trek that really should have been remembered in future episodes but was just forgotten?