^See Inside Star Trek
pp. 206-7. In Herb Solow's words:
Coon, an ardent reader of science fiction since he was a child, in his haste to create a story, had inadvertently based part of is script on a short story that had been written by Fredric Brown. [Researcher] Kellam [DeForest]'s assistant Joan Pearce, who reviewed, analyzed, and wrote the research reports on all Star Trek scripts, recognized the story. She remembers that when she advised Coon of the problem, his reaction was a horrified "Oh my God!" Joan has absolutely no doubt he was unaware he had "lifted" the material. But Coon had transgressed, and there was no way we could shoot the script without buying a plagiarism lawsuit.
Gene Coon and I met with Bernie Weitzman and Ed Perlstein and formulated a plan. Business Affairs would call Brown, tell him Star Trek would like to buy his story, and offer a fair price. Brown was thrilled to have one of his stories on Star Trek and accepted the deal. We never did tell him that the script had already been written.
Of course, the other difference between that and the other two is that Brown had no actual involvement in writing the episode. "The Slaver Weapon" and "Tin Man" were both written by the actual authors of the works they were adapted from.
(There's also TNG's "Where No One Has Gone Before," written by Diane Duane and Michael Reaves as a loose adaptation of Duane's TOS novel The Wounded Sky
and then profoundly rewritten by the TNG staff until it bore almost no resemblance to Duane and Reaves's script. But I didn't count that because the source material was a Trek story, not a story from a different SF universe. And "Slaver" was the only Trek episode adapted from another long-running
SF continuity. Brown's "Arena" and Bischoff & Bailey's Tin Woodman
were both, as far as I know, standalone works rather than parts of series.)