M'Ress has been manning the communication station throughout the episode. However in the stock overhead bridge shots, Uhura is at Communications.
Both times, Nored is alone when she confronts the Vendorian. Standard protocol (as noted in the novelization) and common sense when dealing with a shapeshifter would be to have teams of at least two security officers at all times, to prevent a substitution.
When "McCoy" responds to Spock's question about the possibility of him making an error, Spock briefly turns a much brighter shade of orange/yellow. Later on the bridge when Spock advises surrender, the same basic shot is used and the same coloring error repeats.
When the alien renders McCoy unconscious and returns to talk to Nored, "McCoy"'s hair is briefly a much lighter shade of brown, matching Lt. Nored's.
Granted, McCoy has just been nerve-pinched and rendered unconscious, but it seems impossible that he wouldn't realize there was a third bed in sickbay. He's overhauled the equipment there, treated patients, run diagnoses on the beds... but he doesn't notice there's an extra one?
The Vendorian is capable of taking on physical forms, but there's no explanation for how it can convert itself into energy to make a deflector shield. (Alan Dean Foster attempts to explain in the novelization by saying it took on the form of replacement parts, but no such explanation is presented on-screen.)
Kirk says that there used to be only two beds in sickbay, but surely there would be more. At least there were more in the live-action series. Also, how would he know the exact layout of sickbay. For example: McCoy wouldn't know the exact layout of Main Engeneering now, would he? On the other hand Scotty would!
The insignia on Lt. Nored's uniform jumps to the opposite side in one shot as she holds a phaser on "Winston" and Kirk comes into the room.
McCoy: I must have taken a little nap.
Spock: Doctor, you are a man of curious habits, but I have never known you to nap on the laboratory floor.
Romulan Commander: You appear to have a propensity for trespassing in Neutral Zone, Captain Kirk.
Kirk: It was not deliberate, I assure you.
Romulan Commander: It never is.
McCoy: You caught him.
Spock: Acute observation, Doctor.
McCoy: I'm glad to see him under guard, Jim. If he'd turned into a second Spock, it would've been too much to take.
Spock: Perhaps. But then two Dr. McCoys just might bring the level of medical efficiency on this ship up to acceptable levels.
This episode along with "The Lorelei Signal" was released on The Animated Adventures of Gene Roddenberry's Star Trek: Volume 3 for VHS.
Nichelle Nichols doesn't appear as Uhura.
McCoy mentions his daughter Joanna: the first on-screen mention of such a thing although his daughter was referenced in the original Trek script bible.
This is the third time that we saw a race of alien shapeshifters (the first being the Salt Monster in the Original Series episode "The Man Trap", and the second being Garth of Izar in "Whom Gods Destroy," who was taught how to shapeshift by the Izarites). This makes later references about the Federation's unfamiliarity with shapeshifters (Odo is described as unique, and shapeshifters are described as "legendary" in Star Trek VI: The Undiscovered Country.) Banishing this episode from canon doesn't explain the Garth of Izar thing.
Surprisingly adult episode, this is the only episode of the animated series that really presented an actual romance.
The character of Carter Winston, created here, was later used by author John L. Ford in his novel The Final Reflection, which was a early-Federation era novel about the Klingons.