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)
Weird Cousin / Baby Oskar - S4-E11
Continuity mistake: Weird Cousin: When Lila tells Arnold, "Going to a movie together sounds kind of like a date," the way she holds her book changes between shots. (00:01:41)
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)
Love and Cheese / Weighing Harold - S4-E7
Continuity mistake: Love and Cheese: When Arnold asks the man at the CheeseBall Toss for three tries, Eugene in the background has black skin and a red shirt, like Gerald. His shirt is also red in a later shot when Arnold is trying to knock the bottles down. (00:05:52)
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)
Continuity mistake: Ms. Perfect: When the flap on Lila's table falls down, her lunch bag falls over. When the tray of spinach and fish falls on her, it changes position. (00:18:37)
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
Revealing mistake: Gerald's Game: When Arnold and Gerald are playing The King Rules together, as Arnold says, "I'm not going to quit, Gerald," his mouth briefly disappears. (00:09:49)
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)
Gerald's Game / Fishing Trip - S5-E15
Continuity mistake: Gerald's Game: When Arnold asks Gerald if he was in the alley near Dr. Steiglitz's office, there is nothing in the bottom shelf of Gerald's locker. When Gerald puts his book "The King Rules" in his locker, some books appear in the bottom shelf. (00:03:34)
Summer Love (a.k.a. Beach Story) - S5-E9
Continuity mistake: In the scene at the end where "Babewatch" is being filmed, when Arnold drowns and the lifeguards save him, he is wearing his plaid shorts. A shot later, he is wearing his blue swim shorts. (00:22:34)
Timberly Loves Arnold / Eugene, Eugene! - S5-E22
Continuity mistake: Timberly Loves Arnold: When Timberly and Arnold start crossing the street together, Iggy is standing far away from Rhonda and Sid. In the next shot, he is closer to them. (00:02:57)
Timberly Loves Arnold / Eugene, Eugene! - S5-E22
Trivia: In "Timberly Loves Arnold", when Timberly shows Arnold her flower drawing, as Sid says, "Look, it's a flower named Arnold," part of his shirt is invisible in the first frame of the shot. This can only be spotted in slow motion or with pausing, therefore it's under trivia. (00:02:22)
Stuck in a Tree / Rhonda Goes Broke - S5-E10
Revealing mistake: Rhonda Goes Broke: When Rhonda says, "I'm power walking. It's the healthiest way to get home," her mouth disappears four times. (00:21:42)
School Play - S3-E19
Continuity mistake: When Mr. Simmons tells the class about the Romeo and Juliet play, the projector has four legs. When the class is watching the presentation, it has three legs. Then when the kids leave the class, it has four legs again. (00:02:38)
School Play - S3-E19
Continuity mistake: When Arnold is getting his lunch, his tray is empty. When he picks up the cup of green stuff and Helga gets mad at him, a milk carton appears on his tray. (00:00:29)
School Play - S3-E19
Continuity mistake: When Mr. Simmons tells the class that they are welcome to come and try out for the play, there is no cap on the projector. When the kids start to leave, a cap appears on it. (00:03:18)
School Play - S3-E19
Other mistake: After Helga gets mad at Arnold in the beginning, as Helga says, "What an annoying little smarty pants know-it-all," the lunch lady's body is invisible through the glass on the counter, leaving only her upper half visible. (00:01:12)
Timberly Loves Arnold / Eugene, Eugene! - S5-E22
Continuity mistake: Timberly Loves Arnold: When Gerald says, "I could go for some Arnold lemonade," and Arnold pulls him in the alleyway, there is a building with a sign with the word "shoe" on it, written in red. In the next shot, it changes to yellow. (00:04:07)
Stuck in a Tree / Rhonda Goes Broke - S5-E10
Continuity mistake: Stuck in a Tree: When the fire truck shows up at the end, in Arnold's close-up, his sleeves are drastically shorter than usual. (00:10:29)
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.