Revealing mistake: In the pilot episode (?). When old man Barnabas bends over Maggie's bedside to bite her, the strings holding his bald wig in place are visible, including the stubble, and wave of fake black wig hair. (01:19:29)
Audio problem: In Season 1 Episode 2, Victoria says "boo" to a painting but when she visibly says it, we hear the audible boo sound afterwards. It was dubbed poorly. (00:13:44)
Audio problem: In Season 1, Episode 18 there is an audible knocking on Victoria's door prior to Roger walking up to the door and actually knocking. (00:01:57)
Factual error: 1966: Ohrbach's, the fashion house that supplied Dark Shadows with costumes throughout its run, suffered the indignity of seeing its name misspelled as "Orhbach's" in many of the 1966 credit crawls.
Audio problem: 1966: While Constable Carter is grilling Burke Devlin, you can hear crew members talking loudly backstage. In apparent annoyance, "Burke" raises his voice and all but shouts his next line.
Other mistake: In episode 367, there is a continuity glitch. Vicki Winters travels back in time from 1967 to 1795 as the result of a 1967 seance gone awry. Vicki is disoriented and confused by her surroundings and is made to lie down and rest in a bed. There, a granny square blanket is placed over her - the exact same one as seen in earlier 1967 episodes at Sam Evans' house, draped over his sofa; the green granny squares around the perimeter of the blanket are a dead giveaway that it's the same blanket. Either there was a mistake made in that they hoped nobody would notice that this same blanket appears over and over again in the series (it appears again in the series in unlikely places in episodes even after #367, as well), or it's a mistake to try to pass it off that this 170-year-old blanket was somehow still in perfect condition in the future, in 1967, at Sam Evans' place. Either way, it's a mistake.
Other mistake: 1795: A black-and-white cat, supposedly the transformed Joshua Collins, was completely uncooperative in every scene he was in. He always started out on a bed or a chair, but got up and left every time, usually forcing the camera to pull in so that the actors could pretend the cat was still there.
Revealing mistake: 1968: Adam approaches David in the woods, and brushes against a large shrub. The bush teeters and then falls over with an audible crash.
Visible crew/equipment: In the 1795 flashback, as Barnabas confronts Angelique beside his coffin, a crew member in a short-sleeved blue shirt is sitting to the right, and can be seen hastily getting up and moving out of the shot.
Answer: It's probably episode 193 (even though the slate at the beginning says 58). It's the episode with Sam demanding that Roger return his paintings. Three clues from the Dark Shadows Companion and from the comments of Kathryn Leigh Scott and Louis Edmonds all agree that 1) Scott is in the episode and is done for the day prior to Edmonds without his pants; 2) Edmonds is wearing a smoking jacket, and 3) Edmonds is leaning on the mantle (which is actually the cabinet where the liquor is, because the fireplace mantle is too high to lean on and the bench in front of it prevents getting too close to). There are several "breaks" in the lengthy scene where commercial were probably inserted, and all of the shots are above the waist. The previously mentioned episode 54 has Edmonds in a suit and Scott isn't in the episode; episode 54 doesn't have Edmonds or Scott; and episode 86 only has Edmonds in a suit.
Lou Edmonds was a consummate actor and would not forget that he had a lengthy scene to perform. Rather, in Episode 86, Edmonds more understandably forgot that he had to perform one last 15-second scene (a simple toast). He had removed his shoes and pants before he was reminded of the final, brief shot, and he hurried back to complete it.
Charles Austin Miller