School Play - S3-E19
Continuity mistake: Lila's collar is gold throughout the episode. When the girls audition for the part of Juliet and Mr. Simmons says that they were all very good, her collar is white, and her gold button is missing. Then when Mr. Simmons says Lila will be the third understudy, her collar is gold again, but her button remains missing. Later on, when Mr. Simmons announces Lila will take the role of Juliet, her collar is white again, and her gold button is missing again. (00:08:46 - 00:14:26)
School Play - S3-E19
Continuity mistake: When Helga tries to convince Lila to drop out of the play, as she says, "What about that crazy costume you have to wear?", Lila's gold button is missing. It's also missing when Lila says, "I'm sorry, Helga, you're mumbling." (00:15:42 - 00:17:06)
Continuity mistake: Mugged: Throughout the scene with Arnold and Grandma on the bus, Arnold is not wearing his hat. When the creep runs off the bus and the camera shows Arnold and Grandma from outside the bus, Arnold is wearing his hat. (00:11:35)
Beaned / Old Iron Man - S5-E11
Continuity mistake: Beaned: When Arnold is walking Helga to school and she sees some flowers, they are both standing on the sidewalk. Two shots later, they are on the road. Also, when Arnold walks up to Helga, he is in mid-air, and when Helga stands up, she is also in mid-air. (00:05:53)

Hall Monitor / Harold's Bar Mitzvah - S2-E15
Continuity mistake: Harold's Bar Mitzvah: In the beginning, when Stinky asks Harold if he wants to play stickball, there is no wristband on his arm. Harold says he has to go do his lessons, and in the next shot, a wristband appears on Stinky's arm, then disappears. Later on, when Harold comes out of the synagogue, his wristbands are back on his arms. (00:12:03)
Full Moon / Student Teacher - S4-E1
Continuity mistake: Full Moon: Throughout the episode, Harold's eyes change from yellow to white during a few consecutive shots, as well as mid-shot.

Full Moon / Student Teacher - S4-E1
Continuity mistake: Full Moon: When Arnold leaves the school at the beginning, he is carrying a green book. A few seconds later, the rectangle on the cover changes to white, then back to green. (00:03:14)

Full Moon / Student Teacher - S4-E1
Continuity mistake: Full Moon: When Principal Wartz tells Arnold to see him every day after school for the next four weeks, the words on his office door are yellow. About a second later, they change to red, then back to yellow. (00:02:57)

Tour de Pond / Teachers Strike - S2-E18
Continuity mistake: Tour de Pond: When Rex's grandfather walks up to Arnold's Grandpa at the end, his badge has eight squares on it, but in the next shot, it changes to four stripes. (00:11:20)

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
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
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)

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
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)

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)
Answer: No, he did not. He is seen getting off the bus with a suitcase, so he must have not been home for a few days so there is no way he could have known.
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