The scene in which Kirk is buried in an avalanche of tribbles took eight takes to get right. The tribbles were thrown into the hatch by members of the production crew. The crew members were not sure when to stop because they were unable to see the scene. This is why additional tribbles keep falling on Kirk one by one. William Shatner can be seen looking perplexed as to why more tribbles keep falling on him.
To create the one tribble moving on its own, the prop supervisor bought a battery powered toy dog and stripped it down to the mechanical works. Once recovered with fur including the toy legs, the prop moved on camera along the railing on the Enterprise bridge without wires or external assistance. The toy was so noisy all the dialogue in the scene had to be looped with ADR (re-recorded after shooting).
The pile of tribbles near the end of the episode was actually created by gluing tribble props around a large wire frame which Kirk (William Shatner) then stood in the middle of to give the illusion of mass numbers. In reality there were only five hundred tribbles made.
According to David Gerrold's The World of Star Trek, Tribbles would be around the set for some time afterward, allowing for occurrences such as what was mentioned earlier or popping up in various other places as well for some months after the production of the episode.
According to Michael Okuda and Denise Okuda's text commentary on this episode for the second season DVD set, the last fresh footage of the Enterprise was done for this episode. In every episode to follow, the shots of the ship were all stock footage.
During the famous "bar fight", careful observers will note that while tables are broken, all the chairs remain intact. The tables were studio property: the chairs were rented, and if damaged would have to be paid for.
The line in which Spock says that Kirk heard what Baris said, but could not believe his ears, was lifted directly from a Mad Magazine spoof of Star Trek (titled Star Blecch) that had just been published.
This is the first time in the series the Klingon language is mentioned. It is called "Klingonese" in the script and the DVD-subtitles, however actor Michael Pataki began to laugh at the end of the word, and cut himself off before finishing his consonants, so the word is heard as "Klingonee."
In the bar set, recycled from Star Trek: Court Martial (1967), many tribbles were made out of carpet as background. Most visible versions were made from off-cut fur garments as revealed in book to accompany episode. The tribbles that move had mechanical toys placed inside them.
Despite this episode's popularity, producer Robert H. Justman wrote in his book "Inside Star Trek: The Real Story" that he never liked this episode. Justman felt that the humor was too over-the-top and the show became a parody of itself.
William Schallert appeared at one of the earliest Star Trek conventions, finding the rewarding, and also confusing. Schallert recalled encountering many fans in person, who would react by calling and addressing him as Nilz Barris, and at the time he had completely forgotten the name of the character he played.
There is much discussion of tribbles consuming the ship's resources, but no one ever appears to feed them. In fact, only one is shown eating - in the bar. Nor is there any indication of tribbles producing any kind of waste products.
Initially Leonard Nimoy was not a fan of the episode as he felt it to be frivolous. Its deepening reputation as one of the classics of The Original Series as the years went on helped him change his mind.
In some scenes (and if you watch in high definition), a coffee stain is clearly visible on Spock's velour shirt. Leonard Nimoy spilled his cup of coffee during lunch and there were no other costumes available for him.
When the bartender buys the Tribble, and Uhura asks how much HE would be selling them for, he says he "paid 6 credits and would like a "reasonable " profit, say 10%," so he says he will sell them for 10 credits. A 10% markup would be 6.6credits.. not 10.