Corrected entry: This episode holds the absolute record in TZ for having the most actors who also appeared in other episodes. Barney Philips (Gunther) appeared in "Miniature," "A Thing About Machines" and most notably as a Venusian diner counterman in "Will the Real Martian Please Stand Up?" and is one of only two actors who appeared in four episodes (the other being Burgess Meredith). Dick York (Riker) had a memorable role in "Penny For Your Thoughts," while Warren Oates (Jeep Driver) appeared in "The 7th is Made Up of Phantoms." Despite being the main star in this episode, William Reynolds (Fitzgerald) never appeared in any other TZ episode.
Corrected entry: If you look carefully,when Fitzgerald and Riker are talking in hospital hallway right after their comrade has died, you can see it is the same stage, with some differences, which was used for Barbara Jean Trenton's house in the episode "Sixteen-Millimeter Shrine." Note staircase and garden door.
Correction: This trivia, not a mistake. Sets and locations are resused all the time in films - the "Hill Valley" set in the Back To The Future films appears in dozens of other films, notably Gremlins.
Correction: 1. Regarding the supposed record, "Will the Real Martian Please Stand Up? " has five actors who appeared in other episodes: John Hoyt, Barney Phillips, Bill Erwin, Morgan Jones, Bill Kendis. Even that might not be the record, but I don't have time to go through the whole series right now. 2. Jack Klugman and John Anderson appeared in four episodes each, one of them together ("A Passage for Trumpet"). Vaughn Taylor appeared in five. These are all prominent appearances; Jay Overholts was credited for six smaller roles.