Girl Trouble / School Dance - S3-E18
Other mistake: Girl Trouble: When Phoebe says she has costume designs for the Mandans, the Chinook, and Sacagawea, Gerald says, "I could have sworn it was Sacagawea." Then, there is a shot of Helga and Phoebe, but there is a problem with this shot. What we're supposed to see is Helga and Phoebe sitting in their seats at their side of the table. Instead, the background is of Arnold and Gerald's side of the table, while Helga and Phoebe are shown in front of the table, which doesn't make sense. (00:02:31)
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."
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 ★