Dawson Casting: In the books at that time, the Hardys were supposed to be 16 and 17, and while the show itself never explicitly gives ages for them, it's implied in "Mystery of the Jade Kwan Yin" that they're underage. The actors: Shaun was 19, Parker 25.
Fake Southern: In the episode "Game Plan", West Coast native Shaun Cassidy, playing East-Coast-Massachusetts Joe Hardy, speaks with an atrocious Southern/hick accent in an attempt to go temporarily undercover. It backfires painfully.
Edmund Gilbert (Fenton Hardy) was a respected voice actor and character actor in many major TV series. Among many others, he was Baloo in TaleSpin, Robert Cramer in Ben Casey, and General Hawk in GI Joe.
Herb Voland, who played Chief Collig, was better known as General Clayton in the TV series M*A*S*H.
Casey Kasem (Mystery of the Hollywood Phantom) plays a villain impersonating Columbo, but Mr. Kasem has a long resume of tons of voice parts and character parts — most famously, that of Shaggy in Scooby-Doo and Robin in Superfriends.
Lorne Greene (Inspector Stavlin, The Hardy Boys & Nancy Drew Meet Dracula) likewise has a long resume of character roles, including Commander Adama from Battlestar Galactica.
Larry Storch (Mystery of the Silent Scream), best known for F Troop, but again, a long list of character roles.
The Other Darrin: After Pamela Sue Martin quit her role of Nancy Drew in the middle of Season 2, Janet Louise Johnson filled in for her final four (supporting) appearances that season. Afterword, the character was dropped from the show entirely, and Season 3 focused entirely on the Hardy Boys.
Keep Circulating the Tapes: For a while, Seasons 1-2 were all that were on DVD from Universal Studios, and fans relied on illegal bootlegs and downloads to suit their Season 3 needs. However, Shout Factory finally released the last season to DVD in February 2013. Even though the episodes were taken from lower-quality syndicated versions, the fans rejoiced anyway after years of fearing the final season would never see a disc release at all.