Features of a set you never noticed...

[Ian Wallis]

there was machinery inside that was no longer used.

Piggybacking off that (again, realizing it's not exactly what the OP had intended) - Tattletales.  In the original version there were lights surrounding the monitor to indicate when someone in the back ringed in or got the question right, and an additional set of lights around the couple's names just above the scoreboard.  When they ditched the "story" questions in mid'74, the lights just above the scoreboard were never used again.  Occasionally they'd do a camera angle that showed the lights were still there (they weren't unscrewed or anything).

In the ''82 version all the said lights on the rebuilt set were used to indicated a right answer.

[TimK2003]

On Scrabble there were five tile racks, though by the time I was a regular viewer the regular game would always end at a 2-2 tie so there was machinery inside that was no longer used. A recent re-watch of a show from 1993 indicates that they built the set with just four racks to draw from.

I don't know if this qualifies in the way the OP intended, but certainly piqued my interest.

No , that example fits. I thought because  of the gaps in each individual tile rack, I originally thought it was a more intricate single slot of tiles (like the pop-up Lightning Round words on original Password) with every 4th or 5th tile rising up for that word's tile rack.