Continuity mistake: Ms. Perfect: When Patty opens her locker and the jello falls on her, her head is raised above the floor, and there is no jello on her head. In the next shot, her head is on the floor, and there is jello on her head. (00:16:24)
Arnold's Thanksgiving - S3-E14
Continuity mistake: When Helga is backstage preparing for the play, there is an "H" on the left side of her costume. When Phoebe tells her that she's on, she turns around and the "H" is not on her costume, then reappears a moment later. (00:03:32)
Girl Trouble / School Dance - S3-E18
Other mistake: Girl Trouble: When Arnold comes home after his rough day at school, his hair clips through the wall as he walks in the door. (00:04:39)
Arnold's Thanksgiving - S3-E14
Other mistake: When Arnold is in his room putting on his costume, his reflection in the mirror shows his buttons on the left side of his clothes. Then, when Arnold answers the door, his buttons are on the same side; the reflection was not reversed. (00:09:26)
6th Grade Girls/The Baseball - S1-E5
Other mistake: Sixth Grade Girls: When Arnold and Gerald are dancing with Connie and Maria, in one shot, Maria's left foot is incorrectly overlapping the right foot of the girl behind her. (00:07:32)
Other mistake: Mugged: When the people are standing outside Sunset Arms waiting for Arnold to come out of the garage, Oskar is coloured incorrectly: he has grey hair, a grey beard, an orange shirt, a light blue jacket, and white shoes. (00:05:12)
Helga Blabs It All / Harold the Butcher - S3-E1
Other mistake: In the credits, actor Elliott Gould's name is misspelled as "Elliot Gould." (00:22:55)
Hall Monitor / Harold's Bar Mitzvah - S2-E15
Other mistake: In the credits, actor Elliott Gould's name is misspelled as "Elliot Gould." (00:22:59)
Hall Monitor / Harold's Bar Mitzvah - S2-E15
Other mistake: In the credits, Justin Shenkarow is credited twice for voicing Harold. (00:22:59)
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
Revealing mistake: Full Moon: When Harold tells Arnold, "Remember when I said you didn't have any guts because you wouldn't moon Principal Wartz?" his mouth briefly disappears. This is easier to see in slow motion but is still visible at normal speed. (00:11:16)
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)
Full Moon / Student Teacher - S4-E1
Revealing mistake: Full Moon: When Principal Wartz tells Arnold to name the boys who exposed themselves, as Arnold says, "I can't do that, Principal Wartz," his mouth briefly disappears. (00:02:42)
Full Moon / Student Teacher - S4-E1
Other mistake: Full Moon: When Arnold tells Sid, Stinky, and Harold that he was given four weeks' detention, Harold tells him to not rat on them. Arnold says he won't tell, and when he walks away, his hair passes right through Sid's head. (00:03:30)
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 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
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)
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)
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.