Uhura enters the Recreation Room with McCoy and Sulu, but an overhead shot of them inside shows a blonde, blue-uniformed woman instead of black-haired Uhura in her red uniform. Uhura returns in subsequent shots.
The Romulans are flying in Klingon ships. Going by the stardate in Kirk's log, 3183.3, this episode takes place well before The Enterprise Incident (1968) in which the stardate is 5027.3. The latter episode was the first time the Enterprise had ever encountered the Romulans using Klingon ships. And Chekov had not yet been replaced by Arex.
At 17:21, McCoy stands on the bridge. As he stands with Kirk, his comm-badge is evident. When the two-shot changes to a single shot of McCoy, his comm-badge changes to an over-the-shoulder strap for a carrying case. No comm-badge is seen. Back to the two-shot, the strap is gone and the comm-badge returns.
At 19:18, McCoy stands chest to shoulder with Kirk. In the next shot he stands shoulder to shoulder.
Kirk orders a heading of 902 Mark 6. All headings must be between 1 and 360/000. They mark the points of the compass which has 360 degrees with 360 headed to the galactic center. The "mark" covers the "vertical" plane of the galaxy.
Kirk says he discovered the message on his shirt after he put it on. This would not be possible as the message is on his back.
At 2:47 on a close-up of Spock during a time of emergency, a chair on the bridge appears abandoned in the background. Twenty-six seconds later, an overhead shot shows a red-shirted crewman in the chair.
Just before Uhura, Sulu, and McCoy enter the Rec Room, Uhura's collar is colored red. All uniforms at this time have black collars. In other scenes, Uhura's collar is black.
At 6:29, Lt. Arex has the conn, but no one has replaced him at his navigation console.
At 6:29, Lt. Arex is shown to have the conn; however, Spock, Sulu, and Uhura are in the shot also, and all of them have been shown to be higher in the chain of command than Arex.