James Doohan lost a finger while fighting in WWII, and consistently hid his right hand during the series. While changing polarity on the magnetic probe, his hand can be clearly seen, which shows the absence of the finger.
The bypass valve room that Watkins enters consists of re-used pieces of the Yonada control room from "For the World is Hollow and I Have Touched the Sky". The control panel was re-used from the Vians torture chamber in "The Empath".
This is the first and only time a tricorder is shown on the "automatic distress" setting.. In place of the usual data disc storage slot, it has a flashing light panel. Since the storage slot is visible in a previous scene, it must be on a type of swivel, allowing it to be rotated to reveal the panel when the unit is placed on this setting.
The center section of D'Amato's tricorder differs substantially from the standard Starfleet model. Instead of tape discs and a moiré pattern, it features an intermittently glowing white panel and what appears to be a tubular sensor. In deference to D'Amato's specialty, some prop-conscious fans have dubbed this a "geological tricorder."
This episode was used as the background for the Star Trek: Gateways novel One Small Step, by Susan Wright, which elaborates extensively on the story. The mysteries of this episode were used to help tie in the original series with the rest of the Gateways books.
This is the second time Enterprise crewmembers create a resting place for a fallen comrade; it had happened before in the first season's "The Galileo Seven", where crewmembers Latimer and Gaetano were buried. Captain Picard buries Captain Kirk in a similar way on Veridian III in Star Trek Generations.
The trivia items below may give away important plot points.
After D'Amato dies, Kirk uses his phaser to dig a grave for him. This is only the second time on Star Trek where a crewman is buried by the landing party on a planet, the first occurring in Star Trek: The Galileo Seven (1967). Usually, dead crewmen are returned to the ship.
This is the last episode of TOS in which Enterprise crewmembers (Wyatt, D'Amato, and Watkins) die onscreen or close to it. However, in Star Trek: Requiem for Methuselah (1969), Kirk will report in his opening log that three crew members have died of Rigellian fever.