When the creature as Green confronts Yeoman Rand at the elevator, Rand takes two bites from one of the celery-like pieces on the food plate. Moments later in the biology lab, after she's placed the plate in front of Sulu, Green stares down at it and in the close up the same celery pieces appear whole and not partially eaten.
At the beginning of the show, Kirk picks up a bunch of "flowers" (actually some scrub brush) for McCoy to give to Nancy. Kirk then throws them on the ground. The camera pulls back, and the "flowers" are nowhere to be seen. They should be lying on the ground.
When Captain Kirk is talking to Doctor Crater about going aboard the Enterprise for his protection, Kirk's arms are alternately folded/unfolded between shots.
As the salt vampire attacks Kirk, he screams in agony. However, none of the other crewmen who were killed, emitted a scream. Nancy screams upon finding Darnell, which everyone hears, so it's not possible the Darnell might have screamed, but nobody heard him.
Professor Crater identifies human incisor teeth as having once been fangs. This is incorrect. The canine teeth were originally fangs.
As a professional archaeologist, Dr. Crater would never have allowed his living/working quarters to be in one of the ruins he was studying. He would have had Starfleet build a place away from the ruins so they would be kept as pristine as possible.
Sulu discovers Barnhart's body and reports it to the bridge However, it should have been whomever was in command at the time that notified Kirk on the planet. Not Sulu.
When the salt vampire returns to Nancy's form before entering McCoy's quarters, she appears as Kirk's version (with gray hair), not McCoy's memory version.
When Kirk jumps out from behind the rock after stunning Crater with his phaser, the large "stone masonry" structure next to him wobbles when he nudges it, revealing it's fake. However, the rock on planet M-113 may be volcanic rock or rock such as pumice which is lightweight and the stone masonry made of such, and thus, movable.
The "vampire" has lived without salt for a year, yet once prey shows up, it seems to desperately need salt every few minutes. However, it could be stocking up on salt supplies within its body just as a camel stocks up on water, or as humans and many animals store calcium and other rarer minerals into bone tissue.
When Spock is recovering from his attack, he has green blood on his forehead, which is correct for a Vulcan. However, he also has an inexplicably red gash by the blood stain.
When Dr Crater's phaser destroys a rock, you can plainly see the "rock" is actually polystyrene.
During the climatic scene in McCoy's quarters McCoy is shot out of focus.
Beauregard (also known as "Gertrude") the plant on board the ship's Life Sciences section is obviously a man's hand with opposing thumb and four fingers disguised as petals.
When Spock calls out Crater's name, the professor turns and looks behind him, and then he gets stunned from that direction by Kirk. Spock then enters from the same direction that Crater had been looking. Logically, it would have been better if Kirk called Crater and then shot him, or if Spock called him and then Kirk shot him from behind.
When Kirk walks into the turbolift, returning from his mission to the surface of M113, the cameraman's shadow can be seen following him in. And, this shot is used in many additional episodes of the show.
When the plant Gertrude is frightened by Green, its fronds extend and show that it's Bob Baker's hand inside a glove.
The creature, masquerading as Crewman Green, wanders around the ship for quite a while with his phaser on his hip. Inexplicably, no one notices this deviation from the normal practice of returning phasers to the armory after an away mission.
Even though the M113 Creature can assume the appearance of anyone it chooses, it doesn't explain how it could know anyone or anything about anyone it has never seen before (such as recognizing Dr. McCoy or McCoy"s nickname being " Plum " or knowing to speak Swahili to Uhura).
The creature can take on the form of any person. Yet when Kirk, McCoy and Darnell first meet it, they simultaneously see the monster as different people.
When Uhura replied to the call to report to the Bridge, she didn't turn off the intercom button.
When Sulu and Yeoman Rand found the crewman on the floor of Deck 9, killed by the Swahili-speaking creature, his closed eyes flinched when his face was touched.