Continuity mistake: Ms. Perfect: When Lila and her father are eating dinner, Lila's spoon is in her can of beans in one shot. When the angle changes and her father says, "You sure are quiet tonight," her spoon disappears from the can of beans (the flap of her can of beans is also layered under her father's left arm), then reappears in the next shot. (00:19:05)
Other mistake: Ms. Perfect: When Mr. Simmons tells the class, "I have a note from Lila's father," his right sleeve briefly turns green. (00:19:22)
Character mistake: Helga makes Arnold sit in a plate of chocolate pudding as a prank. Arnold then asks Grandpa on the phone to bring him a new pair of pants. When Grandpa arrives, he asks Arnold, "What's that on your pants?" The pudding is not on Arnold's pants; it's on the back of his shirt. (00:06:25)
Girl Trouble / School Dance - S3-E18
Continuity mistake: Besides the fact that when Helga is sharpening her pencil, the desk arrangement changes, one of the students visible when she looks at Arnold is... Helga. This happens because the shot is taken from the episode "Helga's Love Potion."
Weird Cousin / Baby Oskar - S4-E11
Factual error: Stephen Viksten is credited twice for voicing Oskar.
Longest Monday / Eugene's Pet - S2-E4
Continuity mistake: In "Longest Monday" at one point in the episode, Arnold and Gerald run off of a bus and down an alley. Two fifth graders come chasing after them. In the next scene, you see Arnold and Gerald hiding in the sewer under the 5th graders. Arnold then crawls through a very narrow tube, and Gerald follows. You then see the end of the tube and Gerald come out of the it first, and Arnold climbs out last. It would be impossible for Arnold and Gerald to switch places inside the tube because it was way too narrow.
Mud Bowl / Gerald Moves Out - S2-E9
Continuity mistake: When Gerald catches his little sister destroying his cassette tape, the book she is wrapping changes color from blue to green. (00:12:30)
Audio problem: Right after Helga is standing behind the "ARNOLD" on the pavement, when she says "Hey Arnold", her lips move as if she's saying it slowly (this is fixed in the 4th season).
Continuity mistake: When the four boys form their line (just before the adults call out "Arnold"), watch Harold's tooth; it goes from the right side of his mouth (as you look at him) to his left side.
The Little Pink Book/Field Trip - S1-E2
Continuity mistake: On the "Presidents of the United States" sign behind Helga, the word "United" becomes "Nvited"
Door #16 / Arnold as Cupid - S1-E10
Audio problem: At the hat store, when Arnold says "Huh? What are you, nuts?", his lips don't move - but Gerald's do.
Das Subway / Wheezin' Ed - S1-E12
Factual error: There's no way Grandma could have connected two "live" wires with her bare hands without electrocuting herself, and there's certainly no way the wires could have stayed connected.
Arnold's Hat/Stoop Kid - S1-E3
Continuity mistake: When Grandpa is telling Arnold about how how he tried to cover his head when his hair falls out, the camera cuts back to him with his arm on Arnold's shoulder and Arnold has his hat on, even though the whole story is about him losing his hat.
Continuity mistake: Phoebe is going into the school in one scene, but as the camera moves she vanishes while on screen.
Parents Day - S5-E6
Continuity mistake: In the scene where Arnold's parents leave for the last time, Grandma's hair is dark at first, then it becomes purplish-gray.
Continuity mistake: When Lila is finishing her first joke, Rhonda suddenly switches from her French outfit to her "usual" one; also, during the Jell-O incident, her beret disappears and then reappears.
Other mistake: In the closed-captioning, when Arnold and Gerald "touch thumbs" (and you hear that "sproing" sound), the caption says "[glass shatters]"
The Aptitude Test / Oskar Gets a Job - S3-E9
Audio problem: In the comic book shop, Gerald says "Ho. Touche, sir." but his mouth doesn't move at all.
Best Man / Cool Party - S3-E12
Other mistake: In the scene where Arnold meets the gang in the alley way, they give ideas to crash Rhonda's party. When Curly tells his idea to go to the zoo and free all the animals and there are shots of the gang looking at him confused,in two of the scenes, you'll notice that when Stinky and someone else looks at him, the background is at the school, not the alley.
Operation Ruthless/The Vacant Lot - S1-E7
Continuity mistake: In Operation Ruthless, the pedal in Helga & Phoebe's bumper car changes color from black to gray to black between shots.
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.