Hall Monitor / Harold's Bar Mitzvah - S2-E15
Other mistake: Hall Monitor: At the start of the school assembly, when Principal Wartz thanks Helga for her month as hall monitor, Wartz's right arm is atop the lectern, with his fingers over the lectern's front edge. As Wartz gestures toward Helga with his right arm and hand, the fingers of his right hand oddly remain over the lectern's front edge. Then, after Helga announces she may volunteer again, the audience groans, and in the next shot facing Wartz, he has four hands at the lectern's edge. (00:01:35 - 00:02:00)
Hall Monitor / Harold's Bar Mitzvah - S2-E15
Continuity mistake: Hall Monitor: After Phoebe's eyeglasses are stepped on, when she tells Helga, "Out with the old Phoebe, in with the new," Phoebe's belt buckle and two brown pouches have vanished, then reappear in the next shot. Later, while Phoebe sits on the desk during detention, her belt buckle is missing in her close-ups, and it reappears mid-shot as she crosses her legs. Then, when Phoebe's classmates share how they miss the "old" Phoebe, her buckle and the two brown pouches vanish, but they all reappear in a moment. (00:05:05)
Hall Monitor / Harold's Bar Mitzvah - S2-E15
Continuity mistake: Hall Monitor: In the first and last overhead shots of the detention classroom, the "fire" sign, the fire extinguisher, and light switch are not on the wall beside the door, though all are on the wall in other shots during this scene. (00:09:30)
Hall Monitor / Harold's Bar Mitzvah - S2-E15
Continuity mistake: Hall Monitor: While Phoebe is hall monitor, her watch repeatedly disappears and reappears on her wrist, even mid-shot. (00:05:00)
Hall Monitor / Harold's Bar Mitzvah - S2-E15
Continuity mistake: Hall Monitor: When Principal Wartz tells Helga she cannot volunteer again, he removes the "revered badge" and its black crossbody strap from Helga. Helga then selects Phoebe as the next hall monitor. When Phoebe joins them on the stage, Helga is again wearing the crossbody strap with the badge in the wide shot, but it's gone in the next shot as Wartz places it on Phoebe. (00:02:10)
Hall Monitor / Harold's Bar Mitzvah - S2-E15
Continuity mistake: Hall Monitor: During the school assembly, Principal Wartz's necktie has diagonal stripes, and its top knot stripes switch to the opposite direction, then revert to the proper direction.
Hall Monitor / Harold's Bar Mitzvah - S2-E15
Continuity mistake: Hall Monitor: While Helga is hall monitor, her watch repeatedly disappears and reappears on her wrist.
Sid the Vampire Slayer / Big Sis - S5-E3
Other mistake: Sid the Vampire Slayer: While Sid stands in front of the mirror and points his rubber sword at Arnold and Stinky, the two ghoulish vampires, Arnold asks him, "How can we be vampires if we have reflections?" So when Sid is aware of his mistake, Arnold's eyes disappear and reappear in his reflection mid-shot.
Helga Blabs It All / Harold the Butcher - S3-E1
Continuity mistake: Helga Blabs It All: When the boys get ice cream from the Jolly Olly ice cream truck, Arnold gets a chocolate popsicle, and Gerald gets an ice cream cone. Then, when Helga falls into Arnold's attic bedroom, all the boys stare at her, and as Helga backs out of the room, the ice cream cone Gerald holds changes to a chocolate popsicle.
Save The Tree / New Teacher - S2-E1
Continuity mistake: New Teacher: Arnold's classroom door vanishes from its door frame in various shots, then the door reappears in its proper place. That door's doorknob also switches between the right side of the door and the left side a few times.
Arnold Betrays Iggy / Helga and the Nanny - S3-E10
Continuity mistake: Arnold Betrays Iggy: When Arnold goes to Iggy's home near the start, the front doorknob is on the left side of the door, but the next time Arnold goes to see Iggy to bring him the box of chocolates, the front door opens the opposite way, with its doorknob now on the right side of the door. (00:01:20 - 00:06:40)
Other mistake: Snow: After Grandpa turns the street into an ice rink, it cuts to Arnold in the backyard (by the garage) of Sunset Arms where he's been stowing logs in a storage container. When Grandpa shows up in the backyard and tells Arnold, "You got no play ethic," we see the fronts of Green Meats and Art's Shoe Repair behind Grandpa, but that's impossible because Sunset Arms is on the same side of the street as those two stores; we should only see the buildings/stores that are across the street from the boarding house. (00:21:30)
Parents Day - S5-E6
Continuity mistake: During the flashback, when little Arnold says goodbye to his parents, in the next shot of Stella and Miles, the bag's strap is slung over his dad's left shoulder as a crossbody, but mid-shot, the strap switches to his right shoulder with the bag on the opposite side. (00:14:30)
Parents Day - S5-E6
Continuity mistake: When Grandpa enters and exits Arnold's bedroom, the door opens the opposite way in various shots, with the doorknob on either the left or right. Additionally, the light switch beside the door appears and disappears, between shots.
Helga Blabs It All / Harold the Butcher - S3-E1
Continuity mistake: Helga Blabs It All: While Helga hides in the overhead ventilation shaft, the vent cover has either four, five, or six slat openings in various shots.
Helga Blabs It All / Harold the Butcher - S3-E1
Continuity mistake: Harold the Butcher: The front door of Mr. Green's meat store has either "Green Meats" or "Green's Meats" printed on the door's window.
Helga Blabs It All / Harold the Butcher - S3-E1
Continuity mistake: Harold the Butcher: Mr. Green's meat store has a shopkeeper bell over its front door that vanishes in some shots, then it reappears. For instance, the bell is gone when Arnold is in the store while Harold sits across the street, but we hear it jingle and then the bell reappears when Harold enters the store.
Helga Blabs It All / Harold the Butcher - S3-E1
Continuity mistake: Harold the Butcher: In some shots of Mr. Green's meat store, its front door handle switches to the opposite side. Such as at the start, when Harold stops in front of Mr. Green's meat store because he's hungry. Also, while Harold sits across the street from the meat store, Green asks Arnold if Harold is still out there, and the store's front door handle is on the wrong side of the door. Next shot, as Arnold exits the store, the door opens properly. (00:19:30)
Helga Blabs It All / Harold the Butcher - S3-E1
Continuity mistake: Harold the Butcher: After Mr. Green informs Harold that his week is up, Harold follows Mr. Green into the store, and the green front door remains open. Mr. Green explains that he's getting ready for the annual meat sale, and as Harold begs to keep working in the store, we can see in the background that the store's green front door has vanished. The open green front door is back when Harold exits the store. (00:19:05)
Helga Blabs It All / Harold the Butcher - S3-E1
Continuity mistake: Harold the Butcher: When Mr. Green tells Harold his week is up and that Harold is done paying him back, both of their aprons have a white oval patch with "Green Meats" printed on it. As the scene continues, the white oval patch vanishes from Harold's apron entirely, and Mr. Green's apron patch loses its white background. (00:18:40)
Answer: The show is based in present day times. Plus most cars aren't old fashtioned, I recall seeing a limo and not to mention a cherry picker, etc.