Other mistake: In the closed-captioning, when Arnold and Gerald "touch thumbs" (and you hear that "sproing" sound), the caption says "[glass shatters]"
The Little Pink Book/Field Trip - S1-E2
Other mistake: Field Trip: When Lockjaw goes back into the water after Harold throws his ice cream at him, there is a shot of Arnold looking sadly in Lockjaw's direction, and he is behind the wall like he should be. Two shots later, he is mistakenly placed in front of the wall instead of behind it. (00:15:28)
The Little Pink Book/Field Trip - S1-E2
Other mistake: Field Trip: When Arnold is telling Grandma about Lockjaw, right before Grandma gets up from the couch, her left foot is meant to be on the floor, but it is instead overlapping the couch. Then, when she marches up to the couch while saying, "Like he's some sort of common criminal," her right foot overlaps the couch, and Arnold is kneeling on the backrest of the couch instead of the seat as he should be. (00:18:01)
The Little Pink Book/Field Trip - S1-E2
Other mistake: Field Trip: When Grandma uses the grappling hook to climb the wall, she falls to the other side, and she clips through the grappling hook. (00:19:17)
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.
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: 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)
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)
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)
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: 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)
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)
Other mistake: Ms. Perfect: After Lila answers Mr. Simmons' question about the inventor of the telegraph, as Phoebe says, "But I knew that," the book on Nadine's desk is overlapping the back of Phoebe's chair. (00:14:07)
Other mistake: Ms. Perfect: When the girls sabotage Lila's lunch, as Helga pours out Lila's milk from her thermos, the milk crosses an invisible line and disappears. (00:16:56)
Other mistake: Ms. Perfect: When Lila and her father are eating dinner, as Lila's father says, "You sure are quiet tonight," the flap of Lila's can of beans is layered under her father's left arm. (00:19:04)
Other mistake: Ms. Perfect: When Mr. Simmons tells the class, "I have a note from Lila's father," his right sleeve briefly turns green. (00:19:22)
Arnold Saves Sid / Hookey - S2-E7
Other mistake: The clock in the police station looks like it says it's after 5:00, but school is in session.
The High Life / Best Friends - S2-E10
Other mistake: Best Friends: When Arnold walks across the table during the fashion show, a small chunk of the spotlight vanishes mid-shot. (00:22:13)
The High Life / Best Friends - S2-E10
Other mistake: Best Friends: When Gerald is first revealed during the fashion show, the curtains are transparent. They are never transparent in any other shot. (00:22:26)
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.