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)
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."
School Play - S3-E19
Continuity mistake: When Arnold is getting his lunch, his tray is empty. When he picks up the cup of green stuff and Helga gets mad at him, a milk carton appears on his tray. (00:00:29)
School Play - S3-E19
Other mistake: After Helga gets mad at Arnold in the beginning, as Helga says, "What an annoying little smarty pants know-it-all," the lunch lady's body is invisible through the glass on the counter, leaving only her upper half visible. (00:01:12)
School Play - S3-E19
Continuity mistake: When Mr. Simmons tells the class about the Romeo and Juliet play, the projector has four legs. When the class is watching the presentation, it has three legs. Then when the kids leave the class, it has four legs again. (00:02:38)
School Play - S3-E19
Continuity mistake: When Mr. Simmons tells the class that they are welcome to come and try out for the play, there is no cap on the projector. When the kids start to leave, a cap appears on it. (00:03:18)
School Play - S3-E19
Continuity mistake: In the auditorium, when Mr. Leichliter claps and starts talking to Mr. Simmons, the location he is standing in changes three times. (00:05:19)
School Play - S3-E19
Continuity mistake: In the auditorium, when Mr. Leichliter says, "Indeed, it is I," there is no exit door on the wall to the left of him. In the next shot, an exit door appears on the wall. Later on, in a wide shot of the auditorium, the door is gone, and there are now double exit doors on the perpendicular wall. (00:05:22 - 00:05:57)
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)
School Play - S3-E19
Audio problem: Before the play begins, when Mr. Leichliter tells Mr. Simmons, "It's going to be a fiasco," his mouth doesn't move until the shot changes mid-sentence. (00:19:12)
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
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
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
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)
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
Revealing mistake: Student Teacher: When Olga says, "But, nothing, baby sister!" before giving Helga detention, Rhonda and Brainy disappear for one frame. (00:17:17)
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.
Big Gino / Jamie O. In Love - S4-E2
Other mistake: In the cafe, after Charice speaks, when Gerald says "Aloha, Gladiators on Ice", there are three places where Gerald's mouth disappears.
Big Gino / Jamie O. In Love - S4-E2
Continuity mistake: During "Jamie O. in Love," in the very last scene, Jamie drives up in a car whose front license plate reads "Killer." When they leave, the back license plate reads "Killr." There is no space for a letter to have possibly fallen out (not that a letter can fall off of a license plate).
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.