Summer Love (a.k.a. Beach Story) - S5-E9
Revealing mistake: When Helga is sitting on the boardwalk talking to her locket, as she starts to lay down, she is layered over part of the boardwalk that is in front of her. Her shadow is also layered over it. (00:12:24)
Summer Love (a.k.a. Beach Story) - S5-E9
Continuity mistake: When the seagulls start flying above Arnold and Summer, they are both holding hands. In the next shot, they're not. (00:11:26)
Summer Love (a.k.a. Beach Story) - S5-E9
Continuity mistake: When Helga sprinkles bread crumbs around Arnold and Summer, six seagulls come and fly above them. In the next shot, the shadows of the seagulls are visible, but there are only two seagull shadows. Then when both look up at them, there are only five seagulls flying above them instead of six. (00:11:26)
Summer Love (a.k.a. Beach Story) - S5-E9
Continuity mistake: When Arnold talks to Helga after overhearing her, he tells her that he heard everything she said. As Helga says, "You did?", the lifeguard stand that was behind her suddenly isn't there. Then when Arnold and Summer walk away from her, they are suddenly a lot further away from the lifeguard stand than they were before. (00:11:47 - 00:12:12)
Summer Love (a.k.a. Beach Story) - S5-E9
Continuity mistake: When Arnold walks up to Helga after overhearing her, a surfboard suddenly appears behind Helga, which wasn't there before. A few shots later, it disappears. (00:11:42 - 00:12:07)
Monkeyman! / Buses, Bikes and Subways - S5-E1
Revealing mistake: Monkeyman!: When Arnold tells the kids that Monkeyman saved him, as he says, "He went after the two guys who were trying to mug me," he moves his right hand, and it's layered over Helga's hair. (00:01:25)
Monkeyman! / Buses, Bikes and Subways - S5-E1
Continuity mistake: Monkeyman!: When Sid tells the kids that Gerald should tell the legend of Monkeyman, Arnold is squinting. In the next shot, he's not. (00:01:41)
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)
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)
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)
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)
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)
Helga's Boyfriend / Crush on Teacher - S2-E14
Other mistake: Crush on Teacher: When Arnold and Gerald are walking to school and Arnold gets distracted by the birds and flowers, as he says, "Look at the flowers," his eyelids are not colored in. (00:14:37)
Helga's Boyfriend / Crush on Teacher - S2-E14
Other mistake: Crush on Teacher: When the kids are playing baseball and the baseball hits Arnold, as he says, "I'm fine, Miss Felter," his eyelids are not colored in. (00:13:02)
Revealing mistake: In Arnold's nightmare, when Bob points out that a stork is flying off their roof, his legs are incorrectly layered behind his cape. (00:12:50)
Grandpa's Packard / Phoebe's Little Problem - S5-E19
Audio problem: Phoebe's Little Problem: When Harold wets his pants at the end, Stinky can be heard saying, "Harold wet his pants," but in the shot, he is actually laughing. (00:11:39)
Door #16 / Arnold as Cupid - S1-E10
Other mistake: Arnold as Cupid: When Oskar gives Arnold breakfast, besides the fact that the objects move from the left side of the tray to the right between shots, look at the glass of orange juice as Oskar lowers the tray to Arnold. Despite Oskar never rotating the tray around, the glass of orange juice glides itself over to the top left corner of the tray. Very weird. (00:14:34)
The High Life / Best Friends - S2-E10
Other mistake: Best Friends: When Arnold walks across the table during the fashion show, a small chunk of the spotlight vanishes mid-shot. (00:22:13)
The High Life / Best Friends - S2-E10
Other mistake: Best Friends: When Gerald is first revealed during the fashion show, the curtains are transparent. They are never transparent in any other shot. (00:22:26)
The High Life / Best Friends - S2-E10
Continuity mistake: Best Friends: During the fashion show, in the close-up of Arnold showing off the antennas on his costume, the curtains that he came out of are missing. They are also missing in the shot after Arnold tells Gerald, "Just smile and move, okay?" (00:22:22 - 00:22:50)
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.