These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Angst? What Angst?: Third Season. Last Kiss of Summer has Joe broken up and going on a revenge rampage to avenge the love of his life, who was killed in a car wreck as he was driving; she dies in his arms. Two episodes later, Joe's making out and flirting with other women as if nothing's happened, and the fiance' is never mentioned again.
Big Lipped Alligator Moment: Jaclyn Smith makes a small appearance in The Mystery of the Hollywood Phantom as herself playing her character on Charlie's Angels. She ends up kissing Frank for some reason.
The actor playing Joe Hardy, Shaun Cassidy, gave us these during the course of first and second season, using the show to highlight his singing career: Da Doo Ron Ron, That's Rock & Roll, and Hey Deanie.
Harsher in Hindsight: The 2nd season (1978) episode "Arson & Old Lace"...where an arsonist torches a tall skyscraper, not only trapping Joe Hardy and a small child in an office room, but also trapping Frank Hardy and Nancy Drew in the penthouse, with an un-working elevator & the access stair to the roof welded shut. After helping panicked office workers find the stairs through heavy smoke and flames, Joe delays his escape to follow a cry for help & finds a trapped child. He shields a child from an explosion with his own body (the explosion blows out a window and a good part of the outer wall — the shot of this, taken from outside & ground level, looks disturbingly like something is crashing through the building), then convinces the child to jump with him out of the window, as the fire has cut off all escape routes. Post-9/11, this episode is now hard to watch: people on the upper floors of the WTC were not only trapped by the raging fires below them, but couldn't use the access stairs to the roof, as it'd been sealed shut. Many of those jumped to their deaths in an effort to escape the fire — this makes the child's wide-eyed trust in Joe's assertion that they'd be OK heart-wrenching in hindsight. On top of that, WTC security guards had stayed behind, helping trapped people find their way to the exit stairs — guards who lost their lives due to their heroic delay of their own escape. The episode had only been meant as a cheesy ripoff to The Towering Inferno, but still...
He Really Can Act: Shaun Cassidy & Parker Stevenson, two good-looking, easy-going actors with one-dimensional characters who mostly quip back and forth in a show that's only bait for teenage girls, right? Then we get Third Season. Hoooo, brother, do we get Third Season. Last Kiss of Summer has Shaun sinking his acting teeth into the dark side of Joe Hardy with a vengeance, portraying a revenge-obsessed, bereaved man who tramples over everything & everyone to bring his fiance's killers to justice. On top of that, throughout Third Season, Parker gives us a doubting & conflicted Frank Hardy, who is constantly put into roles of gaining someone's trust & then breaking it (usually in a way that traumatizes the victim); Game Plan brings this to a head, as the show implies that Frank might have betrayed his brother and the federal sting operation in order to run away with the Big Bad, a woman he's show to truly love, if she had just told him the full truth about her escape plans.
a scene where Frank and Joe are talking on the beach, a microphone can very clearly be seen jutting into the scene on the right side.
The Hardys inexplicably develop amnesia between second and third season — they meet Harry Hammond in the second season episode Arson & Old Lace, an FBI investigator assigned to arsons burning down L.A. office buildings. They run into him again (exact same character) in Last Kiss of Summer...which also takes place in L.A....but neither Hammond nor the Hardys seem to know or recognize each other when they run into each other following the death of Joe's fiance, even though Harry makes specific comments in Arson about personally knowing their dad, Fenton.
Even worse, at end of the episode, it changes back to Hammond knowing Fenton after all, as Fenton has evidently been out in L.A. to apply for a job with the Justice Department all along, and tells his boys that he's been hired as a trouble-shooter for Harry.
Mysterious Fate of Flight 608: the stock footage very obviously showing the evacuation of the plane shows male crew members helping the evacuation...but in the episode, the male crew members are all knocked out and unconscious. The only males awake are Frank and Joe Hardy, who are obviously NOT the ones in the stock footage.
Mystery of the Hollywood Phantom: Joe & Frank find headless photographs of themselves as part of a kidnapping/extortion plot. Close-ups of the photos show different photos from shot to shot, even within the same scene.
Mystery of the Jade Kwan Yin:
The boat that the Hardys are searching is at the dock, but the stock footage of a boat explosion shows the boat being far, far out at sea and a completely different make/model, at that.
The jade statue that the Hardys are looking at changes color twice in the same scene, going from a realistic dark jade green to an obviously light fake-plastic green.
Joe states he can't make out the engraving on the bottom of the jade statue — which, in the shots on both versions of the statue, is perfectly clear Asian lettering — and Frank makes a rubbing to "make it clearer". The rubbing is totally illegible.
After stating they're going to check for fingerprints on the statue, a couple seconds later the Hardys let Callie Shaw just take the statue and handle it; Callie isn't wearing gloves. Worse, Frank does the same thing. The only one wearing gloves to protect against fingerprint contamination is Joe...so the show can't claim ignorance of procedure as an excuse.
Creatures Who Came On Sunday:
The Brothers Hardy are supposedly in New Mexico, but Frank is looking at a map of Montana to get directions.
Downtown Justice, New Mexico looks exactly like downtown Bayport, MA...which looks exactly like downtown River Heights.
Voodoo Doll Despite showing stock footage of Bourbon Street and the French Quarter to open the episode, the set looks nothing like New Orleans' French Quarter, and is simply recycled from the "Dracula" episode.
Life on the Line, the last show of the series, very much has the feel & look of the writers throwing up their hands and just not caring. The script and plot are incomprehensible (the Mob is headed by a Russian?), the edits jump from one scene to another with no real links between them, and things happen for no apparent reason, with the plot being resolved by a giant Ass Pull.
House on Possessed Hill The Hardy Boys are in a small town called "Circle Hills", which is stated to be a ways away from Bayport; they rescue a girl who's supposedly psychic. When they show the outside of the girl's house — the show uses a very recognizable stock shot of the Hardys' home in Bayport.
Whenever the Hardy Boys are driving on an out-of-the-way road, the same stock shot is used, over and over, no matter what the location is supposed to be. You see it first in The Disappearing Floor for the outskirts of Bayport, then again in Creatures Who Came on Sunday (supposedly in Mexico), then in etc., etc. The show even uses it for the episodes set in ''Hawaii'!