The Mugato was called The Gumato in the original script. But DeForest Kelley kept mispronouncing it so it was changed. The closing credits still name the creature as The Gumato.
First of two appearances of Booker Bradshaw as Dr. M'Benga, the expert in Vulcan physiology. Second appearance in That Which Survives (1969).
(At around 17 mins) Is the first clear close-up ever of the Sick Bay panel. The vertical scales are, from left to right: Temperature - left scale in °F and right in °C -, Brain - K3 (unknown unit)-, Lungs - no units, but it seems to measure FRC (Functional Residual Capacity) in liters, Cell Rate - no units -, Blood - Q5 (or possibly Qs; perhaps pressure; corrected to %O2 in some mock-ups/merchandise) - and Blood - T2 × 10 (Blood transverse relaxation time - ms ×10). Center symbols: Top Circle "Respiration", second Circle "Pulse" then two legends: Adjust for Normal, Recorder.
The inclusion of both Fahrenheit and Centigrade is odd.
The Star Trek Universe has been known to tackle societal, political, environmental, and other types of issues throughout the history of the franchise. This one tackled the Vietnam War head-on, not only specifically pointing out the "20th-Century brush wars on the Asian continent", but also as portraying the Federation and the Klingon Empire as superpowers using an otherwise peaceful world as pawns in their struggle for power (a direct allegory of the Cold War at that time, between NATO and the Red Bloc).
Nancy Kovack's character (Nona) displays her navel, despite the folklore that broadcast standards censors prohibited showing that part on a woman.