Hall Monitor / Harold's Bar Mitzvah - S2-E15
Other mistake: Hall Monitor: During detention, the class helps Phoebe realise how much she has changed. When Helga wipes the window so Phoebe can see her reflection, in the first shot, Phoebe's reflection is not backwards as it should be. (00:10:45)
Hall Monitor / Harold's Bar Mitzvah - S2-E15
Other mistake: Hall Monitor: At the start of the school assembly, when Principal Wartz thanks Helga for her month as hall monitor, Wartz's right arm is atop the lectern, with his fingers over the lectern's front edge. As Wartz gestures toward Helga with his right arm and hand, the fingers of his right hand oddly remain over the lectern's front edge. Then, after Helga announces she may volunteer again, the audience groans, and in the next shot facing Wartz, he has four hands at the lectern's edge. (00:01:35 - 00:02:00)

Runaway Float / Partners - S2-E20
Other mistake: Runaway Float: After Nick talks about redesigning the parade float, Bob asks the kids who wants to ride on the float. Arnold then tells them it's great the way it is, and when the kids start saying, "Me! Me!", there is an animation glitch where part of Robert's head and hands get stuck in the air for about a second. (00:06:01)

Back to School / Egg Story - S4-E8
Other mistake: Back to School: After Phil completes his long division on the chalkboard and it cuts to a shot of Arnold, the shot is flipped, with Arnold's hair being the most notable giveaway. (00:04:43)

Back to School / Egg Story - S4-E8
Other mistake: Back to School: After Principal Wartz tells Phil he will find a classroom for him, it cuts to a shot inside Mr. Simmons' classroom. If you look at Sid in this shot, the lower half of his body is cut off. Phoebe's right foot is also layered over him. (00:03:58)

Helga's Locket / Sid and Germs - S5-E7
Other mistake: Helga's Locket: When Helga climbs to the top of the boarding house and looks at Arnold and Grandpa through the window, as Arnold tells Grandpa, "It's embarrassing and weird," Helga's left sleeve is incorrectly coloured pink instead of white. (00:09:02)

Helga's Locket / Sid and Germs - S5-E7
Other mistake: Helga's Locket: When Arnold and Grandpa are in the basement and Arnold says that they need the Shiska-Bob Stick, a small portion of the brown paper bag on the workbench is missing. (00:10:06)
Helga's Locket / Sid and Germs - S5-E7
Other mistake: Helga's Locket: When Helga first sneaks into the boarding house, Grandpa is shown trying to open the locket. When the camera pans over to Helga and stops, a small part of Grandpa is still visible; then he disappears mid-shot. (00:03:25)
Other mistake: In the closed-captioning, when Arnold and Gerald "touch thumbs" (and you hear that "sproing" sound), the caption says "[glass shatters]"

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.

Sid the Vampire Slayer / Big Sis - S5-E3
Other mistake: Sid the Vampire Slayer: While Sid stands in front of the mirror and points his rubber sword at Arnold and Stinky, the two ghoulish vampires, Arnold asks him, "How can we be vampires if we have reflections?" So when Sid is aware of his mistake, Arnold's eyes disappear and reappear in his reflection mid-shot.
Other mistake: Snow: After Grandpa turns the street into an ice rink, it cuts to Arnold in the backyard (by the garage) of Sunset Arms where he's been stowing logs in a storage container. When Grandpa shows up in the backyard and tells Arnold, "You got no play ethic," we see the fronts of Green Meats and Art's Shoe Repair behind Grandpa, but that's impossible because Sunset Arms is on the same side of the street as those two stores; we should only see the buildings/stores that are across the street from the boarding house. (00:21:30)

Helga's Masquerade / Mr. Green Runs - S5-E2
Other mistake: Mr. Green Runs: When Green tells Arnold he can't be himself and needs to come up with some fancy words, Arnold is wearing a "Vote Green" button. Then, just as Green says, "I gotta get outta here," Arnold's button now reads "Voet Green" with Vote misspelled.

Part-Time Friends / Bio Square - S2-E19
Other mistake: Bio Square: While Phoebe and Stinky are telling the class about their science fair experiment, just as Phoebe shares that it was in honor of her hero, "I.P. Pavlov," watch as Stinky's eyes move off to the side of his head, then return to normal.

Big Gino / Jamie O. In Love - S4-E2
Other mistake: In the cafe, after Charice speaks, when Gerald says "Aloha, Gladiators on Ice", there are three places where Gerald's mouth disappears.
Benchwarmer / Cool Jerk - S1-E11
Other mistake: The names for the Green and Yellow teams' players are the same.
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.
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)
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)
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 ★