Other mistake: When we first see Arnold walking Helga home after school, as Arnold says, "I never meant for this to happen," Arnold and Helga are briefly duplicated. As the shot continues, Arnold's backpack duplicates repeatedly. (00:11:22)
Continuity mistake: Helga makes Arnold sit in a plate of chocolate pudding as a prank, leaving a huge chocolate pudding stain on the back of his shirt. Then, when Arnold picks up the gift box from Grandpa and returns to the cafeteria, the chocolate pudding is gone from his shirt but reappears later on when Arnold and Helga are at Slausen's Ice Cream. (00:12:37)
Character mistake: Helga makes Arnold sit in a plate of chocolate pudding as a prank. Arnold then asks Grandpa on the phone to bring him a new pair of pants. When Grandpa arrives, he asks Arnold, "What's that on your pants?" The pudding is not on Arnold's pants; it's on the back of his shirt. (00:06:25)
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.