Helga's Masquerade / Mr. Green Runs - S5-E2
Audio problem: Helga's Masquerade: When Helga and Lila are at Slausen's Ice Cream, as Helga tells Lila, "About the way you're always going around acting so nice to people," her mouth stays closed until the word "acting." (00:03:21)
Arnold Visits Arnie / Chocolate Boy - S5-E12
Audio problem: Arnold Visits Arnie: In the country store, when Lulu tells Arnold to get some of the apple butter on the shelf, Arnold says, "Sure," but his mouth doesn't move. (00:05:41)
School Play - S3-E19
Audio problem: Before the play begins, when Mr. Leichliter tells Mr. Simmons, "It's going to be a fiasco," his mouth doesn't move until the shot changes mid-sentence. (00:19:12)
Helga Blabs It All / Harold the Butcher - S3-E1
Audio problem: Helga Blabs It All: When the boys hear the ice cream truck from outside, they all start running and saying, "Ice cream," but none of their mouths are moving. (00:08:48)
Audio problem: When Arnold says, "It's almost like Dino left his whole life in my hands," as Grandpa responds with, "Not his whole life," his mouth doesn't move until the word "life." (00:06:58)
Monkeyman! / Buses, Bikes and Subways - S5-E1
Audio problem: Monkeyman!: When Gerald finishes telling the legend of Monkeyman, Stinky can be heard saying, "That was beautiful, Gerald," but his mouth doesn't move. (00:02:32)
Monkeyman! / Buses, Bikes and Subways - S5-E1
Audio problem: Monkeyman!: When Gerald begins to tell the legend of Monkeyman, as he says the word, "Monkeyman," it looks as if his mouth only says the word, "Man," and it never says the "N" sound at the end. (00:01:50)
Arnold Visits Arnie / Chocolate Boy - S5-E12
Audio problem: Arnold Visits Arnie: During the hayride, Arnold tells Hilda that she has a little straw in her hair. When he says, "Let me get it for you," his mouth does not move; it just stays in a smile. (00:09:14)
Save The Tree / New Teacher - S2-E1
Audio problem: New Teacher: When Helga gives directions to the kids to prank Mr. Simmons, she says, "Everyone know the drill?" and they all put their hands in. Arnold can be heard saying, "Alright," but he was never present in the scene. (00:13:47)
Magic Show / 24 Hours to Live - S1-E18
Audio problem: Magic Show: During Arnold's magic show, the kids start chanting, "Make Helga disappear!" When Helga says, "Alright you bunch of clods," they stop, but their voices can still briefly be heard. (00:03:28)
The Aptitude Test / Oskar Gets a Job - S3-E9
Audio problem: Oskar Gets a Job: After Oskar comes home after delivering the newspapers, as Arnold asks him, "Did you deliver the newspapers?", his mouth says, "Did you," but it never says, "deliver the newspapers?" (00:22:08)
Helga's Parrot / Chocolate Turtles - S4-E6
Audio problem: Helga's Parrot: After Arnold accuses Lila of flicking his ear, as he says, "Well, somebody flicked it," his mouth only says, "Well." It never says, "somebody flicked it." (00:08:38)
Helga's Show / The Flood - S3-E15
Audio problem: The Flood: When the kids go into room 315 looking for Mr. Simmons, Arnold calls out his name, and his mouth does not move. (00:20:24)
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.