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)
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)
Continuity mistake: Ms. Perfect: The red strings attached to Lila's table and chair are only there as Lila is getting pranked; they are not visible in any other shot of the table. Also, when Rhonda pulls the string attached to the table, the string attached to the chair isn't there. (00:18:32)
Continuity mistake: Ms. Perfect: When the girls prank Lila, the table has its flap down, there are four chairs around it, and there is nothing on it. When Helga walks Lila over to the table, the flap is up, there are only two chairs around it, and a "Reserved" card appears on it. When Lila runs out of the cafeteria, there are four chairs around the table again. (00:18:06 - 00:18:48)
Love and Cheese / Weighing Harold - S4-E7
Continuity mistake: Love and Cheese: When Helga drills the hole into the back of Arnold and Lila's boat, she drills it into the side of it, but when it's shown from inside the boat, it's in the center. (00:09:06)
Love and Cheese / Weighing Harold - S4-E7
Continuity mistake: Love and Cheese: Helga drills a hole in the back of Arnold and Lila's boat, but in all the following shots where the back of the boat is shown, the hole is not visible. (00:09:07 - 00:10:31)
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)
Love and Cheese / Weighing Harold - S4-E7
Continuity mistake: Love and Cheese: When Arnold and Lila's boat is stuck at the edge of the waterfall, Lila says, "Arnold, what will we do?", and when the shot changes, Arnold and Lila's hands both move to a different part of the boat. Lila's braids also go from being in front of her shoulders to behind them. (00:10:15)
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, "Are you implying that I'm not going to be king forever?", his eyebrows duplicate as he blinks. (00:05:16)
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.