Voice Credits:

• This episode was a sequel to the Original STAR TREK series episode "Shore Leave" by Theodore Sturgeon.
• A scene in this episode showed the interior of the Enterprise hanger deck. Visible in the hangar deck was a heavy shuttle (as seen in "Mudd's Passion"), and a long range shuttle (as seen in "The Slaver Weapon").
• In this episode, the date of the Caretaker's death could be read off of the gravestone: "7009th year of operation."
• An accomplished singer, Nichelle Nichols as Lieutenant Uhura, sang in this episode as she had done in a number of original STAR TREK episodes. In this episode, she vocalized for almost 40 seconds.
• This episode's co-writer, Chuck Menville, also wrote another animated STAR TREK episode "The Practical Joker". Chuck Menville, along with Len Janson, also wrote for several other Filmation series from 1969 to 1976.
• There is a page in this site that examines the Hover Robots that were seen in this episode.
• A scene in this episode showed the interior of the Enterprise hanger deck. Visible in the large compartment was a heavy shuttle later to be seen in "Mudd's Passion" and a long range shuttle which was used in the later episode "The Slaver Weapon." Visible at the extreme right of the large hanger deck was a small scout ship (pictured) with a rounded foresection. This was a small scout ship that was designed for the animated series but never used in any episode. This scout ship was also seen in the hanger in "Mudd's Passion".
• The phaser bore is a powerful piece of Starfleet equipment used when a landing party's mission requires tunneling underground. The phaser bore is capable of digging through 20 meters of rock per minute.
• When the commandeered ship's computer briefly cuts off the artificial gravity, the crew are sent floating about the ship. This is another example of scenes which would have cost too much to do in live action.
• The animation sequences featuring the red dragon were apparently reused in the first episode of Filmation's Space Sentinels (1977-78) series, albeit with one less head. The episode entitled "Morpheus, The Sinister Sentinel" just so happened to be written by Len Janson and Chuck Menville the authors of this animated episode.
• In this and other episodes, the sciences insignia erroneously appeared on red uniforms.
• The correct spelling of this episode's co-writer is Janson. All episode guides of the animated STAR TREK universally mispelled his name as Jenson.
• This episode had a visual error: the hangar deck doors opened from one side, while in the animated episode "Albatross" and the original series, the doors open from the center like a sideways clam shell.
• Like in the original series episode "Catspaw", in this episode Kirk and his men are briefly menaced by a giant domestic cat who threateningly howls and swats at them.
• Another visual error: McCoy's tricorder strap disappeared in a bridge scene.
• This episode had another mistake in which Sulu was erroneously shown calmly sitting on the bridge while he was actually imperiled on the planet's surface.
• "Once Upon a Planet" was novelized by Alan Dean Foster in Star Trek Log Three published by Ballantine Books in January 1975. Also novelized in the book was "The Magicks of Megas-Tu" and "Mudd's Passion".