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.
Simmons' Documentary / Big Bob's Crisis - S5-E20
Character mistake: In the credits, Anndi McAfee's name is misspelled as "Anndi McAffee." (00:22:58)
Timberly Loves Arnold / Eugene, Eugene! - S5-E22
Continuity mistake: Timberly Loves Arnold: When Arnold, Lila, and Timberly are in the field together after the baseball game, Lila helps Arnold up and looks at her watch, which she wasn't wearing previously. Then, when Lila starts walking Timberly home, her watch has vanished from her wrist. (00:06:35)
Girl Trouble / School Dance - S3-E18
Continuity mistake: Besides the fact that when Helga is sharpening her pencil, the desk arrangement changes, one of the students visible when she looks at Arnold is... Helga. This happens because the shot is taken from the episode "Helga's Love Potion."
Weird Cousin / Baby Oskar - S4-E11
Factual error: Stephen Viksten is credited twice for voicing Oskar.
Mud Bowl / Gerald Moves Out - S2-E9
Continuity mistake: When Gerald catches his little sister destroying his cassette tape, the book she is wrapping changes color from blue to green. (00:12:30)
Continuity mistake: When the four boys form their line (just before the adults call out "Arnold"), watch Harold's tooth; it goes from the right side of his mouth (as you look at him) to his left side.
The Little Pink Book/Field Trip - S1-E2
Continuity mistake: On the "Presidents of the United States" sign behind Helga, the word "United" becomes "Nvited"
Freeze Frame / Phoebe Cheats - S2-E8
Trivia: Arnold's computer is made by Purdy Electronics; Joseph Purdy wrote the episode.
Arnold's Hat/Stoop Kid - S1-E3
Continuity mistake: When Grandpa is telling Arnold about how how he tried to cover his head when his hair falls out, the camera cuts back to him with his arm on Arnold's shoulder and Arnold has his hat on, even though the whole story is about him losing his hat.
Continuity mistake: Phoebe is going into the school in one scene, but as the camera moves she vanishes while on screen.
Parents Day - S5-E6
Continuity mistake: In the scene where Arnold's parents leave for the last time, Grandma's hair is dark at first, then it becomes purplish-gray.
Best Man / Cool Party - S3-E12
Other mistake: In the scene where Arnold meets the gang in the alley way, they give ideas to crash Rhonda's party. When Curly tells his idea to go to the zoo and free all the animals and there are shots of the gang looking at him confused,in two of the scenes, you'll notice that when Stinky and someone else looks at him, the background is at the school, not the alley.
Operation Ruthless/The Vacant Lot - S1-E7
Continuity mistake: In Operation Ruthless, the pedal in Helga & Phoebe's bumper car changes color from black to gray to black between shots.
Tutoring Torvald / Gerald Comes Over - S1-E13
Continuity mistake: Arnold switches from his left hand to his right hand when taking the test.
Suggested correction: I don't believe it is ever established whether Arnold is left or right-handed in the series. Without confirmation; he may very well be part of the ambidextrous population that uses both. I used to write with both hands myself in school.
I do believe he is ambidextrous. In "The longest Monday" he is using chop sticks with his right hand. Many other episodes he uses his left hand to write.
Except the mistake is saying he switches hands in between shots during the test. Not that one episode he's left handed and the next he's right handed. Yes, the teacher was shown for about 1-2 seconds before the switch happened. But these mistakes should be considered valid as an actual mistake in animation rather than to suggest to the audience that for no reason whatsoever, Arnold switches hands to finish the test, and only does so when not seen.
I'm ambidextrous myself, and I often switched hands when taking tests. It was very useful for preventing hand cramps. It only takes a couple of seconds to switch the pencil to the other hand, so I don't see why Arnold can't do it within that span of time too.
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)
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.
