Shatner, Nimoy, and Doohan were all on the original "The Twilight Zone."
I saw this snip about other actors that had cameos as well.Glenn Close makes an appearance in Hook, dressed as one of the (male) pirates on Captain Hook's ship, as an homage to how many stage productions of Peter Pan had the titular character played by a woman.
Shatner, Nimoy and Koenig have been in Columbo. Although only the first two were murderers, Koenig was a cop.Shatner and Nimoy were also both on separate episodes of "The Outer Limits".
I always enjoy these casting gags, like J. Jonah Jameson ALWAYS being J. K. Simmons, no matter the universe/timeline.Paul Reubens played Oswald Cobblepot's father in both Batman Returns and Gotham.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.