Grandpa's Sister / Synchronized Swimming - S4-E12
Continuity mistake: Synchronized Swimming: When the boys take off their jackets during the swimming tournament, Gerald's boots change from brown to black mid-shot, and when the camera pans over to Stinky, his boots are also black instead of brown. (00:20:45)
Grandpa's Sister / Synchronized Swimming - S4-E12
Other mistake: Synchronized Swimming: Right before the boys jump into the pool during the swimming tournament, Gerald's right eye seems to have lost its color. Right when Sid jumps into the pool, his eye is white again. (00:20:52)
Grandpa's Sister / Synchronized Swimming - S4-E12
Continuity mistake: Synchronized Swimming: When the boys take off their jackets during the swimming tournament, Sid is missing from the group. When the boys dive into the pool, he returns. (00:20:46)
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)
Continuity mistake: When Arnold is sitting in the kitchen with Dino's personal stuff, Grandpa offers him a shrimp cocktail, and Arnold turns his head to look at it. In the next shot, he is suddenly looking inside the box. (00:06:45)
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)
The Aptitude Test / Oskar Gets a Job - S3-E9
Continuity mistake: Oskar Gets a Job: After Arnold comes home after delivering the newspapers for the second time, he runs up to the door, but in the next shot, he somehow ends up at the window. His helmet also disappears between shots. (00:19:37)
Dinner for Four / Phoebe Skips - S4-E5
Other mistake: Phoebe Skips: When Lila tells Helga, "I just didn't feel right about destroying Rhonda's personal property," she blinks, and her right eyelid is not colored in. (00:16:19)
The Big Scoop / Harold's Kitty - S2-E3
Revealing mistake: Harold's Kitty: When Arnold kicks the ball during the kickball game, he starts running, and he is layered over Helga, even though he is supposed to be behind her. (00:12:20)
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)
Helga's Boyfriend / Crush on Teacher - S2-E14
Revealing mistake: Helga's Boyfriend: When Arnold walks up to Helga to tell her that Stinky loves her, he asks if he can talk to her about something, and in the shot of Helga, there is a line at the bottom of the screen going through her legs, cutting them off. (00:08:04)
Operation Ruthless/The Vacant Lot - S1-E7
Revealing mistake: Operation Ruthless: When Helga and Phoebe are chasing Ruth in the bumper car, when Ruth's car stops and Helga looks back at her, Helga's right lower leg is incorrectly layered over the bumper car seat, and Phoebe's legs are colored black. (00:08:11)
Operation Ruthless/The Vacant Lot - S1-E7
Continuity mistake: Operation Ruthless: When Helga and Phoebe's bumper car flies into the air and lands on the hot dog truck, as the people start to run away, watch for one person that disappears from the shot, then reappears, and his shirt changes from green to orange. (00:08:36)
Abner Come Home / The Sewer King - S1-E16
Continuity mistake: Abner Comes Home: When Abner comes back to the boarding house at the end, Arnold starts running towards him, and there is no mud puddle. In the next few shots, a mud puddle suddenly appears for both of them to fall into and roll in. Then in the next wide shot, the mud puddle disappears, and in the last two shots of the episode, it returns once again. (00:11:09 - 00:11:48)
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
Revealing mistake: Buses, Bikes, and Subways: When Helga and Harold are riding the clown bike, a tall clown and two little clowns start chasing them. As the clowns are running, the left little clown is layered over the tall clown's right leg for a few frames, and the right little clown runs right through it. (00:17:39)
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)
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.