Darth Plagueis made his first proper Star Wars appearance in The Acolyte, and a previously vague detail about Palpatine's origin story has been confirmed.
Locutus, the Borg alter ego of Patrick Stewart's Jean-Luc Picard in Star Trek, appeared briefly in First Contact, but the costume was missing something.
Jonathan Frakes may have directed the Star Trek: Next Generation episode that inspired a full season of Discovery, but he chalks it up to pure coincidence.
One Twilight Zone episode about a cruise ship sailing across the ocean has a similar ending to the one in The Lord of the Rings: The Return of the King.