After Kirk feeds the love potion crystals to one of the rock creatures, he jumps away, back towards the others with him as the monster starts to fall in love with him. Next, a shot at 21:45 shows all four of the humanoids together: Spock, Chapel, Mudd and... an unnamed security guard (red shirt, dark brown hair) - NOT Kirk (gold shirt, medium brown hair). There are no security guards on the planet with them: Mudd and Chapel arrived via shuttlecraft, Kirk and Spock beamed down by themselves.
Inconsistency. Christine activates the love potion by absorbing it into the skin of her palms before touching the person she wants to affect. Kirk, on the other hand, feeds a handful of crystals into the mouth of a rock monster, whereby it falls in love with him. If the potion were to work the same way, he would have to break the crystals and absorb the potion in his own palms, then touch the monster. Elsewhere, the potion becomes airborne, affecting the Enterprise crew who breathes it in (leaving it unclear if they touched each other for the potion to start its effect).
When the rock creature throws the crushed shuttle on the ground, U.S.S. printed on the side (bordered by a red ban of color) can be seen. This is typical. N.C.C. is printed above this in larger letters but can't be seen as the vehicle is somewhat crushed; but, when the perspective changes to a distance, NCC can be seen but not the lettering below it, and when the perspective changes back to close, NCC can be seen but still not U.S.S. as was seen earlier.
When the love potion starts losing its effect at 20:21, Scotty leans on the Enterprise railing near M'Ress. In the overhead shot, his Starfleet shirt is blue. In the following close-up, it has changed to its regular red color.
In panning the Shuttle Bay, vehicles on the left have "N.C.C." printed on them while one vehicle on the right shows only "NCC" printed on it. The abbreviation periods are inconsistent.
Mudd calls the potion a liquid at first although they are obviously crystals. Later, however, he tells Nurse Chapel that she has to break the crystal to release the liquid inside.
When Harry steals a shuttle craft and kidnaps Nurse Chapel, Kirk and Spock race to pursue him to the planet surface. They could have saved the effort by beaming Mudd and Christine directly off the shuttle, quickly ending the incident.
At 21:15, while standing with Christine and Harry, Spock's insignia appears on the incorrect side of his shirt.
At 13:23, 15:01, 16:45, M'Ress has an all-black nose, which is considered an incorrectly drawn appearance.
Down on the planet, Mudd has only two crystals left, yet Kirk takes them and visibly throws a handful of them at the monster.
After the love potion is drawn into the air ducts, it appears to affect everyone on the ship except Harry - yet it should have, since he was breathing the same air as the rest of the crew.