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.
Weird Cousin / Baby Oskar - S4-E11
Other mistake: Stephen Viksten is credited twice for voicing Oskar.
Freeze Frame / Phoebe Cheats - S2-E8
Trivia: Arnold's computer is made by Purdy Electronics; Joseph Purdy wrote the episode.
Arnold's Hat/Stoop Kid - S1-E3
Question: Did Harold already know that the Stoop Kid had just left his stoop the first time? He would have known that a while ago due to a large crowd in the city or even in the newspaper.
Answer: No, he did not. He is seen getting off the bus with a suitcase, so he must have not been home for a few days so there is no way he could have known.