School Play - S3-E19
Continuity mistake: In the auditorium, when Mr. Leichliter says, "Indeed, it is I," there is no exit door on the wall to the left of him. In the next shot, an exit door appears on the wall. Later on, in a wide shot of the auditorium, the door is gone, and there are now double exit doors on the perpendicular wall. (00:05:22 - 00:05:57)
School Play - S3-E19
Continuity mistake: In the auditorium, when Mr. Leichliter claps and starts talking to Mr. Simmons, the location he is standing in changes three times. (00:05:19)
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 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
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)
Helga's Boyfriend / Crush on Teacher - S2-E14
Continuity mistake: Crush on Teacher: When the kids are playing baseball and Miss Felter drives by, Harold starts walking to the side of the road, and Helga is standing on the road. In the next shot, Harold is already standing to the side of the road, and Helga is standing on the sidewalk. Helga's mask also disappears. (00:12:42)
Weird Cousin / Baby Oskar - S4-E11
Continuity mistake: Baby Oskar: When Oskar asks Suzie to make him breakfast, there are two baby food containers on the table. Later on, when Suzie is on the phone and Oskar walks back into the kitchen, the containers are gone. (00:16:14)
Weird Cousin / Baby Oskar - S4-E11
Continuity mistake: Baby Oskar: When Suzie says to Oskar, "Tomorrow, tomorrow, tomorrow. That's all I ever hear from you," she sets the baby bottle on the table. In the following shots, the bottle is nowhere to be found on the table. (00:15:23 - 00:15:57)
Weird Cousin / Baby Oskar - S4-E11
Continuity mistake: Baby Oskar: In the beginning of the episode, Suzie is ironing Oskar's pants, and the laundry basket is on the floor beside her. Suzie picks up his pants and lays them out on the ironing board, and in the next shot, the laundry basket is suddenly in her hands, and the pants are in the basket. (00:12:21)
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)
The Aptitude Test / Oskar Gets a Job - S3-E9
Continuity mistake: Oskar Gets a Job: When Oskar overhears Arnold and Suzie talking in the laundry room, he is standing next to the light coming from the room. As he starts to walk upstairs, he is suddenly walking out of the light area. (00:21:25)
The Aptitude Test / Oskar Gets a Job - S3-E9
Continuity mistake: Oskar Gets a Job: When Arnold and Suzie are doing laundry together, as Arnold talks about how Oskar was just fooling them all, he starts walking away from the dryer. In the next shot, he is walking away from the dryer again. (00:21:16)
Grandpa's Sister / Synchronized Swimming - S4-E12
Continuity mistake: Synchronized Swimming: When the boys take off their jackets during the swimming tournament, Gerald's boots change from brown to black mid-shot, and when the camera pans over to Stinky, his boots are also black instead of brown. (00:20:45)
Grandpa's Sister / Synchronized Swimming - S4-E12
Continuity mistake: Synchronized Swimming: When the boys take off their jackets during the swimming tournament, Sid is missing from the group. When the boys dive into the pool, he returns. (00:20:46)
Continuity mistake: When Arnold is sitting in the kitchen with Dino's personal stuff, Grandpa offers him a shrimp cocktail, and Arnold turns his head to look at it. In the next shot, he is suddenly looking inside the box. (00:06:45)
The Aptitude Test / Oskar Gets a Job - S3-E9
Continuity mistake: Oskar Gets a Job: After Arnold comes home after delivering the newspapers for the second time, he runs up to the door, but in the next shot, he somehow ends up at the window. His helmet also disappears between shots. (00:19:37)
Operation Ruthless/The Vacant Lot - S1-E7
Continuity mistake: Operation Ruthless: When Helga and Phoebe's bumper car flies into the air and lands on the hot dog truck, as the people start to run away, watch for one person that disappears from the shot, then reappears, and his shirt changes from green to orange. (00:08:36)
Abner Come Home / The Sewer King - S1-E16
Continuity mistake: Abner Come Home: When Abner comes back to the boarding house at the end, Arnold starts running towards him, and there is no mud puddle. In the next few shots, a mud puddle suddenly appears for both of them to fall into and roll in. Then in the next wide shot, the mud puddle disappears, and in the last two shots of the episode, it returns once again. (00:11:09 - 00:11:48)
Summer Love (a.k.a. Beach Story) - S5-E9
Continuity mistake: When the seagulls start flying above Arnold and Summer, they are both holding hands. In the next shot, they're not. (00:11:26)
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.