Arnold & Lila / Grand Prix - S3-E13
Continuity mistake: Arnold and Lila: Helga writes "Arnold loves Helga" on the wall, then replaces "Helga" with "Lila". In the next few shots, the way the word "Lila" is written changes four times. (00:02:48 - 00:03:21)
Arnold & Lila / Grand Prix - S3-E13
Other mistake: Arnold and Lila: When Rhonda, Lila and Nadine are talking about what Helga wrote on the wall, as Lila says, "It couldn't possibly be true, could it?", she blinks, and her right eyelid is not coloured in. (00:03:12)
Arnold & Lila / Grand Prix - S3-E13
Continuity mistake: Arnold and Lila: When Lila starts talking to Arnold in the cafeteria, the sandwich on Arnold's tray has sandwich bread, but in the next few shots, they change to hamburger buns. (00:04:06)
Arnold & Lila / Grand Prix - S3-E13
Continuity mistake: Arnold and Lila: When Arnold and Lila sit down at the table, the borders of their trays are missing, then return in the next shot. (00:04:20)
Arnold & Lila / Grand Prix - S3-E13
Continuity mistake: Arnold and Lila: Arnold and Lila have milk cartons on their trays when they sit down at the table, but in the next shot, Arnold's milk disappears, and Lila's milk is beside her tray. (00:04:20)
Arnold & Lila / Grand Prix - S3-E13
Continuity mistake: Arnold and Lila: On the bus, when Arnold tells Lila that he doesn't like her like her, Lila is smiling at him. In the next shot, she is suddenly frowning. (00:07:01)
Arnold & Lila / Grand Prix - S3-E13
Other mistake: Arnold and Lila: When Arnold tells Lila that the writing on the wall wasn't him, as Lila says, "That's OK, Arnold," she blinks, and her left eyelid is not colored in. (00:07:16)
Arnold & Lila / Grand Prix - S3-E13
Continuity mistake: When Arnold, Sid, Eugene, and Stinky are arguing about the name of the go cart the name is already painted on. (00:17:20)
Arnold's Thanksgiving - S3-E14
Continuity mistake: When Helga is backstage preparing for the play, there is an "H" on the left side of her costume. When Phoebe tells her that she's on, she turns around and the "H" is not on her costume, then reappears a moment later. (00:03:32)
Arnold's Thanksgiving - S3-E14
Other mistake: When Helga tells Phoebe how much she hates Thanksgiving backstage during the play, the third time Phoebe says, "Helga," the background is of Helga's living room, not backstage. (00:03:38)
Arnold's Thanksgiving - S3-E14
Continuity mistake: When the play finishes and all the kids are standing on stage, Arnold and Helga are holding hands. Arnold looks at Helga, and Helga pulls her hand away. When the shot changes, they are holding hands again. (00:05:17)
Arnold's Thanksgiving - S3-E14
Continuity mistake: When Arnold's family is sitting around the table on Thanksgiving, there is a blue bowl with a spoon in it where Arnold is sitting. A few shots later, the bowl changes to white, and the spoon is gone. In the next shot, the bowl is blue again, and the spoon is back. In the shot after that, the spoon disappears again. (00:06:34 - 00:07:08)
Arnold's Thanksgiving - S3-E14
Other mistake: When Arnold is in his room putting on his costume, his reflection in the mirror shows his buttons on the left side of his clothes. Then, when Arnold answers the door, his buttons are on the same side; the reflection was not reversed. (00:09:26)
Arnold's Thanksgiving - S3-E14
Continuity mistake: When Joey is shown in close-up, his missing tooth looks like it's there but it's covered with something black and his missing tooth keeps reappearing and disappearing in the last scenes.
Suggested correction: Joey never appears in this episode.
Helga's Show / The Flood - S3-E15
Audio problem: The Flood: When the kids go into room 315 looking for Mr. Simmons, Arnold calls out his name, and his mouth does not move. (00:20:24)
Helga's Show / The Flood - S3-E15
Continuity mistake: Grandpa's rubber raft got much bigger between the time he left the greenhouse and the time he arrived.
Phoebe Takes the Fall / The Pig War - S3-E16
Revealing mistake: Phoebe Takes the Fall: When Phoebe is in Helga's room helping her study for the tournament, as Helga says, "Well, let's call it a night," her left side hair is missing, then it appears mid-shot. (00:06:54)
Crabby Author / Rich Kid - S3-E17
Continuity mistake: In "Rich Kid" Lorenzo is introduced as a new student, but he was in a previous episode titled "Arnold's Room."
Suggested correction: Other than the few episodes where a specific time of year is taking place; these episodes are not meant to be taken to be shown in chronological order. Just roughly in the same 4th grade school year.
Suggested correction: I agree with the other correction, but it's also worth noting that in the production order, "Rich Kid" was made before "Arnold's Room." They were just aired out of order, just like many of the other episodes.
Girl Trouble / School Dance - S3-E18
Other mistake: Girl Trouble: When Arnold comes home after his rough day at school, his hair clips through the wall as he walks in the door. (00:04:39)
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."