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.
Olga Comes Home / Sally's Comet - S1-E15
Continuity mistake: In Sally's Comet when Gerald, Arnold, and Grandpa are in the traffic jam on the way to Hawk Mountain the position and type of cars around them change. When we first see the car there is a truck there and no room for them to turn around. When they do turn around then there is room and the truck is gone.
Continuity mistake: The pets OK sign on Arnold's house changes in different episodes.
Tutoring Torvald / Gerald Comes Over - S1-E13
Continuity mistake: In Gerald Comes Over, Gerald says whenever he goes to Arnold's house he always goes straight to Arnold's room and has never seen all of Arnold's house. Also he seems never to have met any one else that lives there, but in previous episodes we have seen him in other parts of Arnold's house and met many of the people that live there.
Suggested correction: Other than the few episodes that take place at a certain time; this series is not meant to be taken in chronological order. Just roughly during the same 4th grade school year.
Continuity mistake: When Helga is talking about Arnold, the computer store she is in front of is on a corner (Phoebe is seen going around the corner) but a few shots later the store is no longer on a corner.
The List/Haunted Train - S1-E8
Continuity mistake: At the beginning of The Haunted Train Grandpa's ice cream changes color from orange to white.