Most people are familiar with the iconic James T. (Tiberius) Kirk from Star Trek. But in the original series episode "Where No Man Has Gone Before" a tomb stone is created for Kirk that shows his name as James R. Kirk.
Who was James R. Kirk and when did his name change?
Straight from Memory Alpha:
According to D.C. Fontana in the introduction for Star Trek: The Classic Episodes 1, when the mistake over the middle initial was discovered, Gene Roddenberry decided that if pressed for an answer on the discrepancy, the response was to be "Gary Mitchell had godlike powers, but at base he was Human. He made a mistake."
Found in the article on Where No Man Has Gone Before and in the section on "Sets and Props".
Then the in universe explanation is that Gary Mitchell forget and the out of universe explanation is essentially Word of God, with the producers saying it's a mistake.
James R Kirk is in fact James T Kirk. The out of universe explanation is that in that episode, there was a production goof. The in universe explanation is that the man who created that tombstone, Gary Mitchell, must have misremembered Kirk's middle initial. Everywhere else in The Original Series, he's referred to as James T Kirk.
Other answers include that "James R. Kirk" was an in-joke between them (the "My Brother's Keeper" novel series), or that "James R. Kirk" was an alternate-timeline version of "James T. Kirk" (Q-Squared). Peter David had run with the idea that the slightly different uniforms and careers (Physicist Sulu?) of "Where No Man Has Gone Before" represented a parallel universe (called "Track A").
Explanations that the uniforms had changed soon after "Where No Man..." are contradicted by Kirk and Helen Noel's uniforms in flashback during "Dagger of the Mind".
This episode was the second pilot for the series. The first pilot, "The Cage," was rejected by NBC. It was more than a year before it actually aired. By then, numerous changes had been made to the characters, storyline, costumes, and appearance of the Enterprise. I have heard that for the Blu-Ray release they actually considered changing the "R." to a "T," but decided against it. This is only one of many plot contradictions, errors, and continuity problems that appeared throughout the series' three year run.
One interesting possible answer to this question can be drawn from a reference in one of the original line of Star Trek novels (which, albeit generally considered non-canon, where often written by people close to the original productions and may give some insight into original plans that were dropped, etc).
In this case, the novel is The Galactic Whirlpool by David Gerrold, which includes a section explaining that "Tiberius" is not part of Kirk's given name, but instead is a nickname that was given to him by one of his Academy instructors and which he later officially adopted in order to remind himself of that instructor's lessons.
If this is accepted as true, then it leaves open the possibility that "R" was the initial of an original middle name that he dropped when adopting the nickname.
