Helga Blabs It All / Harold the Butcher - S3-E1
Other mistake: Helga Blabs It All: When Phoebe goes inside the ice cream truck, notice how the door jumps from closed to open, with no animation at all. (00:08:34)
Helga Blabs It All / Harold the Butcher - S3-E1
Other mistake: Helga Blabs It All: When Helga accidentally ends up in Arnold's room, and the camera pans over to Arnold, he is sitting in mid-air. (00:10:59)
Helga Blabs It All / Harold the Butcher - S3-E1
Other mistake: In the credits, actor Elliott Gould's name is misspelled as "Elliot Gould." (00:22:55)
Arnold's Room / Helga vs. Big Patty - S3-E4
Other mistake: Helga vs. Big Patty: When Helga dreams about boxing Patty, as she steps back from her, she is standing in mid-air for about a second. (00:17:00)
Curly Snaps / Pre-Teen Scream - S3-E6
Other mistake: Pre-Teen Scream: When the kids are sitting on Arnold's stoop in the beginning, there are two shots where Arnold's pants are the same colour as his skin, giving the sense that he has no pants on. (00:12:03)
Sid's Revenge / Roller Coaster - S3-E8
Other mistake: Sid's Revenge: When Sid is telling the police officer that he killed his school principal, Arnold's pants are the same colour as his skin, and his hat is not coloured in. (00:09:13)
The Aptitude Test / Oskar Gets a Job - S3-E9
Other mistake: Oskar Gets a Job: In the beginning of the episode, Arnold and Gerald overhear Oskar arguing with a man in his room. In the close-up of Arnold and Gerald as Oskar says, "You're a big man, aren't you?", Arnold's right eyelid is not colored in. (00:12:17)
The Aptitude Test / Oskar Gets a Job - S3-E9
Other mistake: Oskar Gets a Job: On the second day that Oskar forgets to do his job, Arnold reminds him that he has newspapers to deliver. After Oskar says, "You won't believe what happened," Arnold blinks, and his right eyelid is not colored in. (00:18:56)
Career Day / Hey Harold! - S3-E11
Other mistake: Hey Harold!: When Harold and Patty are dancing together, as Harold says, "They said I didn't apply myself," Patty blinks twice, and her right eyelid is not colored in. (00:17:00)
Career Day / Hey Harold! - S3-E11
Other mistake: Hey Harold!: When Harold's family is talking about Patty during dinner, as Harold's mother says, "We can show her those adorable baby pictures of you," her left eyelid is not colored in. (00:20:39)
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.
Arnold & Lila / Grand Prix - S3-E13
Other mistake: Arnold and Lila: When Rhonda, Lila and Nadine are talking about what Helga wrote on the wall, as Lila says, "It couldn't possibly be true, could it?", she blinks, and her right eyelid is not coloured in. (00:03:12)
Arnold & Lila / Grand Prix - S3-E13
Other mistake: Arnold and Lila: When Arnold tells Lila that the writing on the wall wasn't him, as Lila says, "That's OK, Arnold," she blinks, and her left eyelid is not colored in. (00:07:16)
Arnold's Thanksgiving - S3-E14
Other mistake: When Helga tells Phoebe how much she hates Thanksgiving backstage during the play, the third time Phoebe says, "Helga," the background is of Helga's living room, not backstage. (00:03:38)
Arnold's Thanksgiving - S3-E14
Other mistake: When Arnold is in his room putting on his costume, his reflection in the mirror shows his buttons on the left side of his clothes. Then, when Arnold answers the door, his buttons are on the same side; the reflection was not reversed. (00:09:26)
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)
School Play - S3-E19
Other mistake: After Helga gets mad at Arnold in the beginning, as Helga says, "What an annoying little smarty pants know-it-all," the lunch lady's body is invisible through the glass on the counter, leaving only her upper half visible. (00:01:12)
School Play - S3-E19
Other mistake: When the girls are in the bathroom talking about how Arnold will play Romeo in the play, if you look at the reflection of the paper towels in the mirror, it is not reversed. (00:07:36)
Answer: The show is based in present day times. Plus most cars aren't old fashtioned, I recall seeing a limo and not to mention a cherry picker, etc.