This is the actor Kirkwood Smith ( a fine actor) I must have seen DS9's Things Past a half dozen times before a friend pointed that out to me just today. I knew he was in Year of Hell and TUC, but I totally didn't know it was him as this character. Which makes me a double dumbass.
In STAR TREK III: The Search for Spock, I only noticed it within the last year, or so, that as the Vulcan cuties are hauling Spock's stretcher across Mount Seleya, their hands are actually on TOP of him and not UNDER him, as Common Sense might dictate. What ability is causing that, I tell you true: I haven't a clue. I never knew Vulcans had this sort of magnetic ability in their palms and/or fingertips. But in fairness to me, it's not really "in-your-face," or anything. They're a little distant from the camera, but it's still very obvious, none-the-less ... and I never picked that up!
It could be that his stretcher uses anti-gravs. Anyway, much like not recognizing Kurtwood Smith, it was a long time before I recognize John Larroquette in Star Trek III. And this was during the time that I was a big Night Court fan.
Kirk putting his boots on in 'Wink of an Eye'. That big rock Kirk holds in 'What Are Little Girls Made Of' That Captain Tracey and Simon Van Gelder were played by the same actor.
I saw Shore Leave like a bazillion times when I was young, but it was only last year I noticed the chain on the tiger.
I don't have any outright examples, but watching episodes from the first two seasons of TNG/DS9/VOY reveal more jokes and clever lines than I realized.
In ``A Piece Of The Action'', when Bela Oxmyx is playing pool, he just shoots using any old ball, not paying attention to whether it's the cue ball or not. Most likely, this is because the actor had to shoot something that was nearby and wouldn't screw up the way the scene was blocked and nobody was going to futz about with the balls on the table between takes. But it could be a lovely little bit of characterization: Bela's in charge and if he's going to cheat, he's going to do it and nobody's going to call him on it. Or it could be an even more brilliant bit of world-building: the Iotians have gotten the idea of pool from the Horizon's description and have recreated much of the form, but they've missed an essential part of the game and never suspected it. Or they just wanted the actors to not screw up the blocking.
It took me 20 years (or more) of watching TOS before I finally noticed the lizard wall decoration next to the door in sickbay.
I've gotten into the habit of reading the Memory Alpha articles on each episode after I watch it. I've learned some interesting facts on that site (ex: when a Trek guest star has appeared on multiple episodes as different characters).
I've only recently noticed Kirstie Alley "mouthing" William Shatner's lines in STAR TREK II, before she stands up and delivers her own, about "General Order 12". Interestingly enough, she doesn't move her lips to any of DeForest Kelley's lines ... ((...snicker))
Me watching Battle Lines a few weeks ago: "Hey, it's Mike!" Me watching Pegasus a week ago: "Hey, it's Locke!" Me watching Siege of AR-558 a few months ago: "Hey, Lennier!" It's always fun to watch the episode that features the actor you got to know a lot better in a later series.
That ensigns and crewmen were indistinguishable from one another in TOS (both wore the same uniform without any sleeve stripes). The rank insignias in TWOK. It really wasn't until a second viewing that I figured them out. The original Enterprise sometimes had different nacelles, bridge domes, and deflector dishes in the same episodes of TOS due to earlier stock footage being used featuring different models of the ship.
Not to be pedantic, but Babylon 5 wasn't really a "later series". I checked the appropriate wikis, and Siege of AR-558 aired only one week before Babylon 5 completed its entire run. Um, it turns out that was, in fact, entirely pedantic. Sorry.
There was only one 11-foot "hero" model used for FX shots, but it was modified between the first and second pilots and again before series production began. The 33-inch demonstration model was used for a few crude flyby shots in the original pilot.
There are a couple sequences I never noticed before in THE VOYAGE HOME, probably due to the fact that I barely watch it and they are both the same issue. When the HMS BOUNTY (!!!) is about to fly around the sun, we see a shot of the bridge, facing the viewscreen, as the sun fills the scene with bright, yellow light. Kirk then says, "... we need break-away speed, Mister Sulu," only the "Mr. Sulu" part of the line is muted, for some reason, despite our continuing to see Shatner mouthing it. Later in the movie, there is a scene where the whales are at sea and about to get harpooned. The BOUNTY is approaching from the sky and Kirk asks to see the whaling ship, onscreen. For reasons unknown, Gillian is amazed by this, "... how can you do that?" she blurts out. But, she does it twice. There's an odd cut, just before she delivers the line in close-up, there's a group shot where she says the line, but it's muted. We actually see her saying the line twice. Nimoy should've been more careful and caught obvious instances like this, but he just couldn't be arsed, I guess ...
That sort of postproduction glitch would actually be the responsibility of the film editor, Peter E. Berger. Unless Nimoy worked closely with the editor, as directors sometimes do.
In TNG's Parallels, in which Worf is traveling through the multiverse, Data has blue eyes rather than yellow in one of the scenes. I saw that episode several time before I realized that.