Ascended Fanon: The be-eyepatched Very Old Indy of the Chronicles was originally proposed as a narrator for the opening and closing "bookend" scenes of an Indiana Jones series proper Expanded Universe novel. Lucas rejected the conceit for the novel, reasoning that "no one wants to see an old Indy," only to turn around and "borrow" the idea (right down to the eyepatch!) in his new TV series. As noted above, many fans still agree with his initial assessment.
Sean Patrick Flanery, playing Young Indy, was a decade less Young than his character.
Corey Carrier got in on it as well, albeit unintentionally. When the episodes were recut and put together in more chronological order, episode that were supposed to be chronologically one right after the other (but were filmed years apart) show how much Carrier grew during the series. The first two episodes are Egypt and Morocco, and Carrier looks about a foot taller in the second.
Anthony Daniels (C-3PO in Star Wars) in "Attack of the Hawkmen".
Big Mac in "The Phantom Train of Doom" two-parter/telemovie was played by the actor who started out on the colony that gets consumed by Aliens before getting hired to captain a certain rouge-colored mining ship.
Among the episodes being prepped when the series was cancelled were an adventure starring Even Younger Indy (aged 5), and Indy's first meetings with Abner Ravenwood and Rene Belloq (the latter in a story also involving a certain crystal skull).
River Phoenix was first offered the role of Young Indy, to continue from Last Crusade. He declined, as he didn't feel television had anything to offer his career.
Paul Freeman (Belloq!) as real-life explorer Frederick Selous in two episodes.
Actors would often play multiple roles in the series; sometimes this could get a little confusing. The most iconic example is Vic Tablian, who already had two different roles in Raiders (Barranca and the Monkey Man). Here he plays villain Demetrios in the two-parter "The Curse of the Jackal" and an Armenian saboteur in "Istanbul, September 1918".