Arnold Saves Sid / Hookey - S2-E7
Other mistake: Arnold and Gerald's lockers don't have padlocks, like most of the others do.
Suggested correction: Like you state, most of the other lockers have padlocks, which means that some don't. You can even see some lockers without padlocks in the beginning. This is not a reason as to why Arnold and Gerald's lockers having no padlocks is a mistake. It more has to do with the fact that their lockers in particular should have padlocks, because they usually have them in other appearances. Edit: While I do agree with what I said, I now think that changing the mistake's wording would have been a better option than adding a correction, as Arnold and Gerald's lockers not having padlocks is still a mistake on its own.
Steely Phil / Quantity Time - S2-E12
Continuity mistake: The digits on Arnold's clock get bigger and change shape while he's talking to Grandpa.
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.
Runaway Float / Partners - S2-E20
Continuity mistake: Runaway Float: When Arnold stands up on the bike before jumping onto the parade float, Eugene's hair disappears mid-shot. (00:10:11)
Coach Wittenberg / Four-Eyed Jack - S2-E16
Continuity mistake: Coach Wittenburg's anchor tattoo changes color from red to purple in between shots.
Suggested correction: No, his tattoo is always red.
The High Life / Best Friends - S2-E10
Deliberate mistake: In "The High Life", there is a scene where Stinky submerges his head in a tank of water. He delivers the line, "Gee willickers, it's still ticking", and everyone somehow understands what he is saying with his head underwater.
Suggested correction: Nobody responds to what Stinky says, so there's no reason to assume that everyone understood him.
Save The Tree / New Teacher - S2-E1
Continuity mistake: In the beginning of 'Save The Tree', the tree ladder is a distance above the ground. Later on in the episode, when Arnold and his friends go to the tree house for the last time, the ladder is practically touching the ground.
Save The Tree / New Teacher - S2-E1
Continuity mistake: When Harold eats Mr. Simmons' tomato sandwich, there is a red stain from the tomato on Harold's desk. This stain disappears in later shots.
Runaway Float / Partners - S2-E20
Continuity mistake: The kids' teacher in this episode is Miss Slovak, who was their teacher in the first season, but in the first episode of this season they had a new teacher, Mr. Simmons.
Suggested correction: Other than the few episodes that take place at a specific time; this series was not meant to be taken in chronological order. Just roughly during the same 4th grade school year.
Part-Time Friends / Bio Square - S2-E19
Continuity mistake: The kids' teacher in "Bio Square" is Miss Slovak, who was their teacher in the first season, but in the first episode of this season they had a new teacher, Mr. Simmons.
Suggested correction: Other than the few episodes that take place at a specific time; this series is not meant to be taken in chronological order. Just roughly during the same 4th grade school year.
Coach Wittenberg / Four-Eyed Jack - S2-E16
Factual error: When the kids are bowling for the first time, the placement of the pins is wrong, and when Arnold is bowling there are 11 pins.
Save The Tree / New Teacher - S2-E1
Continuity mistake: In New Teacher when the principal is telling the class about their new teacher the students switch seats between shots.
Steely Phil / Quantity Time - S2-E12
Continuity mistake: Chinese Checkers is supposed to be with ten pieces per side, not six.
Freeze Frame / Phoebe Cheats - S2-E8
Continuity mistake: The next night, Phoebe's bed clothes switch back and forth between her nightgown and her dress.
Eating Contest / Rhonda's Glasses - S2-E13
Continuity mistake: In the last scene, Rhonda is seen through the bus's rear window as sitting in the back of the bus - but just before that, she's talking to the girl with the frizzy hair in a seat closer to the front.