Dr. Smith tells Will he won't be able to vote until he's 21. While that was true at the time the show was filmed it wasn't true for the time the show took place in. At the time of production no one could have known that in 1971 the 26th Amendment would pass and lower the voting age to 18.
The display screen seen on the alien ship during the opening sequence is the same one seen on the Seaview and the flying sub on Voyage to the Bottom of the Sea (1964) and with the same exact pattern(s).
Guest star Edy Williams (Non) left the world of family-fare television behind after marrying film producer Russ Meyer in 1970 (divorcing in '75). Williams' career veered into such adult fare as Beyond the Valley of the Dolls (1970) and The Seven Minutes (1971), as well as a Playboy magazine pictorial in March 1973.