The Aptitude Test / Oskar Gets a Job - S3-E9
Plot hole: Mr. Simmons asked that each student put their name on the test folders, but the folders already had names on them when he handed them out.
Love and Cheese / Weighing Harold - S4-E7
Plot hole: Love and Cheese: Arnold and Lila go to the Cheese Fair together in this episode. However, the episode "Operation: Ruthless" also takes place at the Cheese Fair, and it is stated in that episode that the Cheese Fair is an annual event. It can't be that this episode takes place a year later, as the series takes place during the same fourth-grade school year.
Helga's Locket / Sid and Germs - S5-E7
Plot hole: Helga's Locket: When Grandpa tells Arnold about his grandfather's Shiska-Bob Stick, he says that no one has ever touched it since his grandfather's last barbecue on July 16, 1926. However, in the season 3 episode "Grandpa's Birthday," Grandpa says that his grandfather died in 1921 while he looks at the photo album. (00:09:55)
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.