Revealing mistake: While Kirk watches, the machine fires at the Enterprise. As Spock is knocked backwards, watch his hands as he goes forward. They automatically go into a certain, almost practiced position. Then a moment later, the Communications officer is reporting to Spock. As she does, another shot hits the ship. She and just about all the crew react to the jolt a second before the camera and lighting crews simulate the hit. (00:26:40)
Revealing mistake: Accounting for some effects being re-done by modern digital formatting, when the Enterprise approaches the drifting Constellation, as it rotates, you can see a star pass through the solid part of the Constellation's saucer.
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.