Tour de Pond / Teachers Strike - S2-E18
Continuity mistake: Tour de Pond: When Rex notices that Arnold's boat is winning and starts running, the red and yellow squares on his grandfather's badge change to red and black. (00:10:18)
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)
Tour de Pond / Teachers Strike - S2-E18
Continuity mistake: Tour de Pond: When Gerald puts the boat made out of the block of wood in the fish tank, the sail is pointing towards the right, but when the boat sinks, it's pointing towards the left. (00:02:18)
Tour de Pond / Teachers Strike - S2-E18
Continuity mistake: Tour de Pond: When Arnold makes a boat out of a block of wood, there is a blue cup on the newspapers with gray scissors and another orange object in it. In the next shot, the cup becomes green, and the scissors and other object become pink. As the scene continues, they change colours a few more times. The pencils also become the same colour as the newspapers. (00:02:11 - 00:03:49)
Simmons' Documentary / Big Bob's Crisis - S5-E20
Continuity mistake: Simmons' Documentary: Rhonda wears a black top and red skirt on the day of the documentary, but in one shot, she is wearing her regular outfit. She is also wearing her regular outfit in one shot when the class is watching the documentary. (00:06:12 - 00:11:16)
Simmons' Documentary / Big Bob's Crisis - S5-E20
Revealing mistake: Simmons' Documentary: When Phoebe asks Mr. Simmons, "If we're required to do book reports, do we need to choose a book?", her mouth disappears twice. This is easier to see in slow motion, but it's still visible at normal speed. (00:04:27)
Simmons' Documentary / Big Bob's Crisis - S5-E20
Continuity mistake: Simmons' Documentary: When Mr. Simmons explains the documentary and the kids leave the classroom, Arnold is holding a purple book. In the next shot, it changes direction while in his hand. (00:01:50)
Back to School / Egg Story - S4-E8
Continuity mistake: Egg Story: When Arnold shows the little chick to the class, Phoebe is sitting behind Harold, but she is suddenly standing in the next shot. (00:22:30)
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: 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
Continuity mistake: Synchronized Swimming: During the swimming tournament, the boys take off their jackets beside the pool. When the boys jump in the pool, they disappear. (00:20:55)
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)
Arnold & Lila / Grand Prix - S3-E13
Continuity mistake: Arnold and Lila: When Lila starts talking to Arnold in the cafeteria, the sandwich on Arnold's tray has sandwich bread, but in the next few shots, they change to hamburger buns. (00:04:06)
Arnold & Lila / Grand Prix - S3-E13
Continuity mistake: Arnold and Lila: When Arnold and Lila sit down at the table, the borders of their trays are missing, then return in the next shot. (00:04:20)
Arnold & Lila / Grand Prix - S3-E13
Continuity mistake: Arnold and Lila: Arnold and Lila have milk cartons on their trays when they sit down at the table, but in the next shot, Arnold's milk disappears, and Lila's milk is beside her tray. (00:04:20)
Arnold & Lila / Grand Prix - S3-E13
Continuity mistake: Arnold and Lila: Helga writes "Arnold loves Helga" on the wall, then replaces "Helga" with "Lila". In the next few shots, the way the word "Lila" is written changes four times. (00:02:48 - 00:03:21)
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
Audio problem: Helga Blabs It All: When the boys hear the ice cream truck from outside, they all start running and saying, "Ice cream," but none of their mouths are moving. (00:08:48)
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.