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.
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.