Benchwarmer / Cool Jerk - S1-E11
Other mistake: The names for the Green and Yellow teams' players are the same.
Ghost Bride/Gearold VS Jameo - S5-E25
Continuity mistake: In the scene where they are running from the second ghost bride, they go into a crypt and close the doors. One of the kids puts a shovel through the handles to keep it shut. In the next shot, it's gone. In the shot after that it's back.
Continuity mistake: Harold's age is inconsistent throughout the series. Harold has his Bar Mitzvah in "Harold's Bar Mitzvah", establishing that he is 13 years old, and in "Hey Harold!", he tells Patty that he was held back a couple years, which is why he is still in fourth grade. However, in "Helga on the Couch", he is shown in the same preschool class as the other kids, and in "The Journal", he is shown playing in the park with the other kids, and he is a baby just like all of them.
Suggested correction: It's possible that Harold wasn't the only student held back, even though he's older than his classmates. Just a thought.
So? How does that justify him being in the same preschool class as the other kids?
He was probably held back in preschool but was not mentioned.
Fair enough, but he is also in "The Journal" which took place before the kids were in preschool. He is shown playing with the other kids as a baby, and there is no sign of an age difference, even though he is supposed to be 4 years older than them at most. All the other main kid characters are around 9 years old in the fourth grade. It's highly unlikely that Harold would be around those kids as a baby.
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Beaned / Old Iron Man - S5-E11
Continuity mistake: When Arnold reads to Helga, the book title says "GEO-" and "GRAPHY" (two words); the next day, it says "GEOGRAPHY" (one word).
Stuck in a Tree / Rhonda Goes Broke - S5-E10
Other mistake: In the scene after the garbage dumpster scene, Buckley's lips disappear briefly while he's talking.
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Love and Cheese / Weighing Harold - S4-E7
Continuity mistake: Lila was pretty much entirely underwater in a "standing" position when Arnold rescued her, how come suddenly the water only reached her knees?
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Continuity mistake: In "Miss Perfect", Helga, Rhonda, Sheena, Phoebe, and Nadine are cooking up ways to make Lila's (the new girl) life miserable. In one shot, Rhonda is wearing her new blue outfit. When the scene cuts to the boys and Lila, and then back to the five girls, Rhonda is wearing her standard red sweater and black pants. For the rest of that day, she is back in her new outfit.
The Aptitude Test / Oskar Gets a Job - S3-E9
Continuity mistake: In "Oscar Gets a Job", Oscar goes around asking people for money. When he talks to Ernie, you can see that there is a phone and a light to the right of the door. When Arnold confronts Oscar in the hall, you can see Ernie's room (7). The phone has disappeared.
Magic Show / 24 Hours to Live - S1-E18
Continuity mistake: The boarding house has a downstairs bathroom, but in "Gerald Comes Over", there's only one bathroom and it's upstairs.
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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)
Simmons' Documentary / Big Bob's Crisis - S5-E20
Other mistake: Big Bob's Crisis: When Bob faints at the restaurant and first appears as an angel, his eyes become the same colour as his skin twice. (00:13:58)
Timberly Loves Arnold / Eugene, Eugene! - S5-E22
Other mistake: Timberly Loves Arnold: When Harold is leaving the field after the baseball game, he vanishes while on screen without fully leaving the frame. (00:06:15)
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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)
Stuck in a Tree / Rhonda Goes Broke - S5-E10
Other mistake: Stuck in a Tree: In Arnold's imagination, when the gang passes the huge tree on the conveyor belt, notice how huge Sid is compared to Rhonda. It looks as if Sid was meant to be in front of Rhonda, but was instead improperly layered behind her. (00:07:43)
Continuity mistake: Ms. Perfect: When Lila and her father are eating dinner, Lila's spoon is in her can of beans in one shot. When the angle changes and her father says, "You sure are quiet tonight," her spoon disappears from the can of beans (the flap of her can of beans is also layered under her father's left arm), then reappears in the next shot. (00:19:05)
Other mistake: Ms. Perfect: When Mr. Simmons tells the class, "I have a note from Lila's father," his right sleeve briefly turns green. (00:19:22)
Other mistake: When we first see Arnold walking Helga home after school, as Arnold says, "I never meant for this to happen," Arnold and Helga are briefly duplicated. As the shot continues, Arnold's backpack duplicates repeatedly. (00:11:22)
Continuity mistake: Helga makes Arnold sit in a plate of chocolate pudding as a prank, leaving a huge chocolate pudding stain on the back of his shirt. Then, when Arnold picks up the gift box from Grandpa and returns to the cafeteria, the chocolate pudding is gone from his shirt but reappears later on when Arnold and Helga are at Slausen's Ice Cream. (00:12:37)
Arnold & Lila / Grand Prix - S3-E13
Audio problem: Arnold and Lila: Near the start, Arnold tells Stinky, "I like Lila, but I don't like her, like her." The camera cuts to a different angle before he says the last "like her," but his mouth doesn't lip sync the words. (00:01:21)
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Sid the Vampire Slayer / Big Sis - S5-E3
Other mistake: Sid the Vampire Slayer: When Arnold and Sid are in the bathroom, there are two close-up shots of Sid as he speaks to Arnold. In them, the reflection of the door handle is not reversed in the mirror. Also, the style of the door handle in the reflection is different. (00:05:08)