Mistakes in the Enterprise?

Similar framing can be seen in the briefing room and engineering. While those rooms might also be on the outer portions of the ship, another explanation might be that the Enterprise has such bracing through-out much of the interior of the ship.

The ship experiences stress during high warp, and sudden shocks during combat, and so has heavy bracing.

 

In this shot maybe, but I think that's because the wall behind Spock is just some hastily thrown-up flats. To the right of his shoulder is the edge of the beam on the angled window wall, which seems abruptly cut short.
If the correct wall flat were used there would indeed be a support strut, as in here:
http://tos.trekcore.com/hd/albums/1x05hd/theenemywithinhd190.jpg

 

I like the displays-as-windows, though of course there would be some real windows for the rare occasions where you need to look outside but the displays aren't working.

And then it reminded me of a webcomic from a decade ago: the lead character is sitting on a hill admiring nature's beauty when suddenly the sky turns to static. He shouts out, "Hey God, the sky's down!" There is a bit of grumbling, and then the sky full of fluffy clouds resumes. Except all the colors are wrong.

 

According to Doug Drexler on the Ticonderoga Star Trek set tour a couple of years ago, those bulkheads were there both to imply placement on the ship and to give the ship a subtle nautical feel. And Drexler talked to Matt Jefferies about the TOS sets a LOT, so he ought to know. You can see those curved supports in the briefing room, Kirk's quarters, and in Engineering. All of them imply that they're close to the exterior of the ship.

 

The beams also helped create the illusion of a ceiling.

Other shows did it too: the Bewitched house set has a ceiling beam that starts at the front door, passes over the living room staircase, and goes all the was back to the patio doors. All three points have a vertical post to support this wooden beam, which supposedly supports the ceiling. But the only time we ever saw the ceiling was once when Samantha was lighter than air and floated up to it ("Samantha is Earthbound", 1972). For that one episode, I'm sure they had to build the set piece.