Parents Day - S5-E6
Other mistake: After Grandpa knocks Bob into the gelatin, Arnold and Grandma run up to Grandpa. In the next shot, Arnold's left side hair has five spikes instead of four like it should. (00:21:55)

Parents Day - S5-E6
Continuity mistake: During the triathlon, Miriam and Grandma climb up the ropes of the brick wall, but when they both get to the top, the ropes have vanished. (00:20:36)

Parents Day - S5-E6
Continuity mistake: When Miriam and Grandma are climbing up the ropes of the brick wall, there is no one standing on the balance beam above the gelatin, but when they ride the zip lines, Bob and Grandpa are suddenly standing on it. (00:20:42)
The Little Pink Book/Field Trip - S1-E2
Continuity mistake: Arnold is talking to his Grandma about the turtle. There is a table in front of them with three items on it: a remote, a magazine, and what looks like food with a cup. In one shot, the items move around at least twice, depending on which angle the shot is.
The Little Pink Book/Field Trip - S1-E2
Continuity mistake: The orange juice container on the table jumps around when Arnold's grandpa is talking to him about his crush. First it's in the middle of the table, then it moves right. Then back and right again.
The Little Pink Book/Field Trip - S1-E2
Continuity mistake: Arnold puts the pink book on top of a bookshelf at an angle. When Helga comes into his room moments later, the book is upright.

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
Other mistake: Stephen Viksten is credited twice for voicing Oskar.

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.
Longest Monday / Eugene's Pet - S2-E4
Continuity mistake: In "Longest Monday" at one point in the episode, Arnold and Gerald run off of a bus and down an alley. Two fifth graders come chasing after them. In the next scene, you see Arnold and Gerald hiding in the sewer under the 5th graders. Arnold then crawls through a very narrow tube, and Gerald follows. You then see the end of the tube and Gerald come out of the it first, and Arnold climbs out last. It would be impossible for Arnold and Gerald to switch places inside the tube because it was way too narrow.

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: 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.
Audio problem: Right after Helga is standing behind the "ARNOLD" on the pavement, when she says "Hey Arnold", her lips move as if she's saying it slowly (this is fixed in the 4th season).

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"

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.
Door #16 / Arnold as Cupid - S1-E10
Audio problem: At the hat store, when Arnold says "Huh? What are you, nuts?", his lips don't move - but Gerald's do.
The Aptitude Test / Oskar Gets a Job - S3-E9
Audio problem: In the comic book shop, Gerald says "Ho. Touche, sir." but his mouth doesn't move at all.

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.
Das Subway / Wheezin' Ed - S1-E12
Factual error: There's no way Grandma could have connected two "live" wires with her bare hands without electrocuting herself, and there's certainly no way the wires could have stayed connected.






Answer: The episode doesn't really elaborate on who the kid was and what his motives were, so its pure speculation as to why he took it. Perhaps the kid could simply want a ball all for himself and decided to just take it, or was simply a rotten kid. The truth is the kid's motives are not important to the story other than it highlights Arnold was having a lot of trouble getting through the list.
Lummie ★