Girl Trouble / School Dance - S3-E18
Other mistake: Girl Trouble: When Arnold comes home after his rough day at school, his hair clips through the wall as he walks in the door. (00:04:39)
Arnold's Thanksgiving - S3-E14
Other mistake: When Arnold is in his room putting on his costume, his reflection in the mirror shows his buttons on the left side of his clothes. Then, when Arnold answers the door, his buttons are on the same side; the reflection was not reversed. (00:09:26)
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)
Arnold Visits Arnie / Chocolate Boy - S5-E12
Other mistake: Arnold Visits Arnie: When Arnold wakes up from his dream and he screams, his collar briefly loses its colour. (00:10:38)
Helga Blabs It All / Harold the Butcher - S3-E1
Other mistake: In the credits, actor Elliott Gould's name is misspelled as "Elliot Gould." (00:22:55)
Hall Monitor / Harold's Bar Mitzvah - S2-E15
Other mistake: In the credits, actor Elliott Gould's name is misspelled as "Elliot Gould." (00:22:59)
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)
Hall Monitor / Harold's Bar Mitzvah - S2-E15
Other mistake: In the credits, Justin Shenkarow is credited twice for voicing Harold. (00:22:59)
Dinner for Four / Phoebe Skips - S4-E5
Other mistake: Dinner for Four: When Arnold and Lila are talking to each other in the beginning, Lila says that she hopes to become a famous ballerina one day if she practices hard enough. In this shot, the stripe on her shirt is incorrectly layered over her gold button. (00:01:26)
Full Moon / Student Teacher - S4-E1
Other mistake: Full Moon: When Arnold tells Sid, Stinky, and Harold that he was given four weeks' detention, Harold tells him to not rat on them. Arnold says he won't tell, and when he walks away, his hair passes right through Sid's head. (00:03:30)
Tour de Pond / Teachers Strike - S2-E18
Other mistake: Tour de Pond: When Arnold, Gerald, and Grandpa are first shown at the Tour de Pond race, their ship is not sitting on the sidewalk, but rather on the edge of it, which makes it look like it’s floating in mid-air. (00:05:02)
Back to School / Egg Story - S4-E8
Other mistake: Egg Story: When Arnold tells Helga, "I know you don't like me, and frankly, you're not my favorite person either," his mouth disappears twice. (00:14:51)
Grandpa's Sister / Synchronized Swimming - S4-E12
Other mistake: Synchronized Swimming: When the boys take off their jackets during the swimming tournament, Arnold's jacket is floating in mid-air, and the left side of Eugene's jacket is also in mid-air. (00:20:45)
Grandpa's Sister / Synchronized Swimming - S4-E12
Other mistake: Synchronized Swimming: During the swimming tournament, when the boys jump in the pool and it cuts to the crowd, Lila's dress and bows are incorrectly coloured blue, and they stay that colour for the rest of the episode. (00:21:04)
Helga Blabs It All / Harold the Butcher - S3-E1
Other mistake: Helga Blabs It All: When Helga accidentally ends up in Arnold's room, and the camera pans over to Arnold, he is sitting in mid-air. (00:10:59)
Helga Blabs It All / Harold the Butcher - S3-E1
Other mistake: Helga Blabs It All: When Phoebe goes inside the ice cream truck, notice how the door jumps from closed to open, with no animation at all. (00:08:34)
Love and Cheese / Weighing Harold - S4-E7
Other mistake: Love and Cheese: When Arnold and Lila grab each other's arms on the boat, part of Arnold's left arm becomes invisible. (00:10:18)
Gerald's Game / Fishing Trip - S5-E15
Other mistake: Gerald's Game: When Arnold is watching Gerald and the other boys play The King Rules through the window, as Gerald says, "Are you implying that I'm not going to be king forever?", his eyebrows duplicate as he blinks. (00:05:16)
Gerald's Game / Fishing Trip - S5-E15
Other mistake: Gerald's Game: When Arnold is watching Gerald and the other boys play The King Rules through the window, as Gerald says, "There are a lot of terms," Arnold blinks, and his eyes are briefly not coloured in. (00:04:58)
Grandpa's Sister / Synchronized Swimming - S4-E12
Other mistake: Synchronized Swimming: Right before the boys jump into the pool during the swimming tournament, Gerald's right eye seems to have lost its color. Right when Sid jumps into the pool, his eye is white again. (00:20:52)
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.