Weird Cousin / Baby Oskar - S4-E11
Continuity mistake: In "Baby Oskar" the apartment number on the door to Oskar and Suzie's apartment appears and disappears multiple times throughout the episode.
Arnold & Lila / Grand Prix - S3-E13
Continuity mistake: When Arnold, Sid, Eugene, and Stinky are arguing about the name of the go cart the name is already painted on. (00:17:20)
Coach Wittenberg / Four-Eyed Jack - S2-E16
Factual error: When the kids are bowling for the first time, the placement of the pins is wrong, and when Arnold is bowling there are 11 pins.
Save The Tree / New Teacher - S2-E1
Continuity mistake: In New Teacher when the principal is telling the class about their new teacher the students switch seats between shots.
Olga Comes Home / Sally's Comet - S1-E15
Continuity mistake: In Sally's Comet when Gerald, Arnold, and Grandpa are in the traffic jam on the way to Hawk Mountain the position and type of cars around them change. When we first see the car there is a truck there and no room for them to turn around. When they do turn around then there is room and the truck is gone.
Continuity mistake: The pets OK sign on Arnold's house changes in different episodes.
Continuity mistake: When Helga is talking about Arnold, the computer store she is in front of is on a corner (Phoebe is seen going around the corner) but a few shots later the store is no longer on a corner.
The List/Haunted Train - S1-E8
Continuity mistake: At the beginning of The Haunted Train Grandpa's ice cream changes color from orange to white.
Helga's Locket / Sid and Germs - S5-E7
Continuity mistake: When Sid decides to be a slob again, he picks up his frog and you can see that he has gloves on. When the following shot shows a close-up of the frog in Sid's hand, the hand holding the frog does not have gloves on.
On The Lam / Family Man - S5-E17
Character mistake: In the wrestling match, the bell rings after the referee counts to 3 - but he's counting while a wrestler is on the mat stomach-first; a pin requires the loser to be on his back.
Parents Day - S5-E6
Continuity mistake: In the tricycle section of the last event, the chinpiece of Helga's helmet keeps switching colors.
Dinner for Four / Phoebe Skips - S4-E5
Other mistake: When Helga and Phoebe leave the girls' bathroom together near the end, Helga pushes the door outward, but the hinges are on the inside, so it should only open inward.
It Girl / Deconstructing Arnold - S4-E9
Continuity mistake: When Eugene breaks into the area with the dogs, you see the pedals of his bike tossed into the air, but the pedals fell off on the other side of the street.
Career Day / Hey Harold! - S3-E11
Continuity mistake: There are more than 16 slips in the hat, but only 16 students in class, and only 12 hands went in to pull out slips.
Big Gino / Jamie O. In Love - S4-E2
Continuity mistake: During "Jamie O. in Love," in the very last scene, Jamie drives up in a car whose front license plate reads "Killer." When they leave, the back license plate reads "Killr." There is no space for a letter to have possibly fallen out (not that a letter can fall off of a license plate).
Steely Phil / Quantity Time - S2-E12
Continuity mistake: Chinese Checkers is supposed to be with ten pieces per side, not six.
Freeze Frame / Phoebe Cheats - S2-E8
Continuity mistake: The next night, Phoebe's bed clothes switch back and forth between her nightgown and her dress.
On The Lam / Family Man - S5-E17
Other mistake: A security guard in the train yard is drinking from a carton of milk depicting the boys. However, they had only been declared missing that afternoon. It is impossible to put out milk cartons that quickly.
Love and Cheese / Weighing Harold - S4-E7
Character mistake: Love and Cheese: When Arnold and Lila are sitting at the table at the Cheese Festival, Arnold asks Lila, "Do you want to ride on the Tilt-To-Whirl?" However, the ride is actually called "Built-To-Hurl." (00:06:59)
Continuity mistake: Ms Perfect: When Lila is wearing her new blue outfit at recess, she wears light blue bows in her braids, but when she is shown in close-up, she is wearing her regular green bows. There is also a small animation glitch with Lila's right bow. (00:15:22)
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.