Something I didn't pick up on when watching the original airings, but noticed while watching re-runs a very long time ago:
At the climax of "Who Mourns for Adonais", Kirk orders the ship to fire on Apollo's temple... a musical cue plays over the sequence, there is the sound of phasers, and toward the end of the sequence, we see flashes of lightning accompanied by the sound of thunder.
At the climax of "The Apple", Kirk orders the ship to fire on Vaal... a musical cue plays over the sequence, there is the sound of phasers, and toward the end of the sequence, we see flashes of lightning accompanied by the sound of thunder.
The full soundtrack music+phasers+thunder seems to play out identically between the two sequences - same music cue, same thunder (I think at the same relative timing). It's like the same almost-complete soundtrack was spliced from one episode to the other (excepting for the dialog, which of course differs between the two episodes).
From a production perspective, what was going on here? No way this was a coincidence. I know that many episodes used "tracked" music rather than an original score for budgetary reasons. But the thunder sounds? Was a partial soundtrack (music+effects) lifted from one episode to the other, and if so, is there something about the production process that creates such an opportunity (or at least did back then)? Especially since there had to be a narrative reason in both episodes for the thunder.