Other mistake: Snow: After Grandpa turns the street into an ice rink, it cuts to Arnold in the backyard (by the garage) of Sunset Arms where he's been stowing logs in a storage container. When Grandpa shows up in the backyard and tells Arnold, "You got no play ethic," we see the fronts of Green Meats and Art's Shoe Repair behind Grandpa, but that's impossible because Sunset Arms is on the same side of the street as those two stores; we should only see the buildings/stores that are across the street from the boarding house. (00:21:30)
Benchwarmer / Cool Jerk - S1-E11
Other mistake: The names for the Green and Yellow teams' players are the same.
Abner Come Home / The Sewer King - S1-E16
Other mistake: Abner Come Home: When Arnold wakes up and starts looking for Abner, as he walks into the kitchen, his hair clips through the doorway. (00:04:14)
Abner Come Home / The Sewer King - S1-E16
Other mistake: Abner Come Home: In Abner's flashback, Grandpa shows him how to get in and out of the door. We see grass outside the door, but what we should actually see are the stairs that lead to the door, not grass. (00:03:08)
6th Grade Girls/The Baseball - S1-E5
Other mistake: Sixth Grade Girls: When Arnold and Gerald are dancing with Connie and Maria, in one shot, Maria's left foot is incorrectly overlapping the right foot of the girl behind her. (00:07:32)
Other mistake: Mugged: When Arnold gets mugged, some of the shots use a flipped background. Note the wall colours, and the sign that says "MARKET," which is reversed. (00:01:16)
The Little Pink Book/Field Trip - S1-E2
Other mistake: When Arnold is reading the diary at the school the other kids ask him to read more. He says "OK, but it is the last page." There are more pages. When he starts to read, then it is the last page.
Other mistake: Mugged: When the people are standing outside Sunset Arms waiting for Arnold to come out of the garage, Oskar is mistakenly drawn with grey hair, a grey beard, an orange shirt, a light blue jacket, and white shoes. (00:05:12)
Door #16 / Arnold as Cupid - S1-E10
Other mistake: Arnold as Cupid: When Oskar gives Arnold breakfast, besides the fact that the objects move from the left side of the tray to the right between shots, look at the glass of orange juice as Oskar lowers the tray to Arnold. Despite Oskar never rotating the tray around, the glass of orange juice glides itself over to the top left corner of the tray. (00:14:34)
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.