Revealing mistake: At the end, a special effects matte flaw causes the rear tip of the starboard side nacelle to vanish briefly as the Enterprise orbits Scallos. (00:45:50)
Continuity mistake: In the 40th Anniversary release of the complete original series, this episode opens with Scotty at the conn, making an extended log entry, wearing his third-season swept-back hairstyle, and it is very obvious that he is visually mouthing several completely different sentences than we hear in the audio. After the opening credits roll, we next see Scotty wearing his more typical relaxed hairstyle with slight bangs. It is apparent that, in remastering and editing this episode, they inserted footage of Scotty from another episode altogether. For whatever reason, it is a glaring mistake of both continuity and audio editing.
Revealing mistake: In the Enterprise lab, as a sped up Spock goes to move away from McCoy and Chapel, McCoy's eyes follow his movements rather than looking at where Spock was supposed to be after he "disappeared."
Answer: Kirk was getting his physical and Dr. McCoy probably turned off communications, because if he hadn't, Kirk would have left and headed straight for the bridge, leaving McCoy irritated.