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
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)

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
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)
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)

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
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)
Grandpa's Sister / Synchronized Swimming - S4-E12
Other mistake: Synchronized Swimming: Right before the boys jump into the pool during the swimming tournament, Gerald's right eye seems to have lost its color. Right when Sid jumps into the pool, his eye is white again. (00:20:52)

Arnold & Lila / Grand Prix - S3-E13
Other mistake: Arnold and Lila: When Arnold tells Lila that the writing on the wall wasn't him, as Lila says, "That's OK, Arnold," she blinks, and her left eyelid is not colored in. (00:07:16)

Arnold & Lila / Grand Prix - S3-E13
Other mistake: Arnold and Lila: When Rhonda, Lila and Nadine are talking about what Helga wrote on the wall, as Lila says, "It couldn't possibly be true, could it?", she blinks, and her right eyelid is not coloured in. (00:03:12)

Dinner for Four / Phoebe Skips - S4-E5
Other mistake: Phoebe Skips: When Lila tells Helga, "I just didn't feel right about destroying Rhonda's personal property," she blinks, and her right eyelid is not colored in. (00:16:19)

The Aptitude Test / Oskar Gets a Job - S3-E9
Other mistake: Oskar Gets a Job: On the second day that Oskar forgets to do his job, Arnold reminds him that he has newspapers to deliver. After Oskar says, "You won't believe what happened," Arnold blinks, and his right eyelid is not colored in. (00:18:56)

The Aptitude Test / Oskar Gets a Job - S3-E9
Other mistake: Oskar Gets a Job: In the beginning of the episode, Arnold and Gerald overhear Oskar arguing with a man in his room. In the close-up of Arnold and Gerald as Oskar says, "You're a big man, aren't you?", Arnold's right eyelid is not colored in. (00:12:17)

Helga's Boyfriend / Crush on Teacher - S2-E14
Other mistake: Crush on Teacher: When Arnold and Gerald are walking to school and Arnold gets distracted by the birds and flowers, as he says, "Look at the flowers," his eyelids are not colored in. (00:14:37)

Helga's Boyfriend / Crush on Teacher - S2-E14
Other mistake: Crush on Teacher: When the kids are playing baseball and the baseball hits Arnold, as he says, "I'm fine, Miss Felter," his eyelids are not colored in. (00:13:02)

The High Life / Best Friends - S2-E10
Other mistake: Best Friends: When Arnold walks across the table during the fashion show, a small chunk of the spotlight vanishes mid-shot. (00:22:13)

The High Life / Best Friends - S2-E10
Other mistake: Best Friends: When Gerald is first revealed during the fashion show, the curtains are transparent. They are never transparent in any other shot. (00:22:26)

Runaway Float / Partners - S2-E20
Other mistake: Runaway Float: After Bob says, "Now this float has to scream 'beeper' at the top of its lungs!", as he walks away from Arnold and Helga, Joey has white skin, blonde hair, and an orange shirt. (00:06:31)

Arnold's Room / Helga vs. Big Patty - S3-E4
Other mistake: Helga vs. Big Patty: When Helga dreams about boxing Patty, as she steps back from her, she is standing in mid-air for about a second. (00:17:00)
Monkeyman! / Buses, Bikes and Subways - S5-E1
Other mistake: Buses, Bikes and Subways: When Helga and Harold are running to catch the bus, as Harold says, "Ow, quit pulling my arm," Helga is frozen at the right side of the screen for a second before she starts moving. Also, her mouth doesn't move when she says, "The bus is leaving, you dumb oaf." (00:13:59 - 00:14:53)
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.
Bowling255 ★