The Aptitude Test / Oskar Gets a Job - S3-E9
Continuity mistake: Oskar Gets a Job: When Arnold and Suzie are in the laundry room, Suzie says that she wishes Oskar would do what he says he's going to do. In the next shot, the washing machine and the dryer switch places. (00:21:10)
Arnold & Lila / Grand Prix - S3-E13
Continuity mistake: Arnold and Lila: On the bus, when Arnold tells Lila that he doesn't like her like her, Lila is smiling at him. In the next shot, she is suddenly frowning. (00:07:01)
Arnold's Room / Helga vs. Big Patty - S3-E4
Continuity mistake: In the locker room, Sid's hair goes from stringy to full and back to stringy.
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.