The show's "romance" between Frank Hardy and Nancy Drew. The actors Parker Stevenson and Pamela Sue Martin had played lovers before in a movie about a teen girl's first time, "Our Time" (also called "The Death of Her Innocence" when it aired on TV).
Third Season episode "Last Kiss of Summer" (aired in 1978) has Emergency!'s Kevin Tighe (aka Roy) playing an undercover federal agent, trying to nail two men who robbed a mail truck in "spring of '72". One of the robbers is "Dobson", played by Mills Watson...who previously played a bank robber in the Emergency! episode "Understanding", which aired in Winter 1973...which, due to fast production schedules for the time, means it was filmed in...yup..."spring of '72".
Parker Stevenson was also a good surfer in real life. One episode, "Wipe Out" had a silly contrived mechanism of Frank Hardy placing in a national surfing competition, just to get the boys to Hawaii and in the middle of a hotel theft ring.
Dawson Casting: Parker Stevenson was in his mid-twenties at the time he portrayed 17-year old Frank Hardy; Shaun Cassidy was a little closer, being 19 to play 16-year-old Joe. The show eventually tried to avert this in third season by having the brothers grow up to take jobs with the Justice Department.
The third season DVD release deleted the opening scene of "Game Plan", which shows Joe Hardy chasing down a bad guy in an airport as a precursor to the plot. Versions in circulation from "other sources" have the scene in question.
Ditto the opening of the 3rd season episode "Scorpion's Sting"; there was a full sequence of Frank running from the cops through the streets of Chicago. The DVD release cuts all that out. However...Netflix's newly-remastered HD stream of the episode restores the full scene in all of Parker's sweaty, heavy-breathing glory.
Assault on the Tower (3rd Season) likewise has an extended opening of Fenton Hardy on the run from The Villain, which was cut from the DVD release. Netflix's remastered HD stream also restored the full scene.
Fake Russian: Third Season Episode "Life on the Line" has The Villain sporting a horrible, Boris-Badenough fake Russian accent...even though he's supposedly the head of "the Mob".
Keep Circulating the Tapes: Likely due to the extensive overuse of David Gates & Bread music, Season Three will probably never see the light of day. It's aired in reruns on Retro TV, the Disney Channel, and Nick's TV Land. Copies of those have been discreetly airing among fans for years.
Averted! Season Three is now available on DVD from Shout! Factory — however, it's the syndication versions only, with missing scenes and bad edits. Disclaimers on the DVD state the eps were remastered from "best available video sources", which is industry code for "the originals no longer exist".
Shaun Cassidy was mostly unknown, appearing in small productions with his mother and in independent films before winning the part of Joe Hardy, which opened his career up. He went on to star in the TV Series Breaking Away and to produce award-winning TV shows such as American Gothic. His Facebook page profile proudly lists that he's a "former boy detective" in honor of that star-making role.
Parker Stevenson had been in films such as A Separate Peace and Our Time before the Hardy Boys, but wasn't well known. Playing Frank Hardy made his career: he went on to star in Baywatch and many other TV-series and movies, and is now a successful professional photographer.
Nancy Drew's actor, Pamela Sue Martin, was replaced mid-season two by Janet Louise Johnson, who looked nothing like the original actress. The change is not noted nor commented on by the other actors, though in Voodoo Doll, when Janet makes her debut, there is a moment where Nancy takes off a mask, and Frank and Joe looked completely shocked & surprised at not having recognized her before this.
Ned Nickerson was originally played by George O'Hanlon Jr for first season, then inexplicably replaced by Rick Springfield in season two AND re-introduced as a completely new character, a young hotshot lawyer that Nancy has supposedly never met before.
Nancy's best friend, George Fayne, also gets replaced mid-series, originally played by Jean Rasey, then changed to Susan Buckner. Again, no explanation and no comments from her close friends.