Arnold Betrays Iggy / Helga and the Nanny - S3-E10
Other mistake: Helga and the Nanny: Near the start, in the shot where Bob leaves the kitchen to answer the door, watch as Helga's mouth opens and closes with no animation whatsoever. (00:12:27)
Arnold Betrays Iggy / Helga and the Nanny - S3-E10
Other mistake: Helga and the Nanny: At the start, Miriam's dress is its usual purple colour, but when Inge arrives and walks into the kitchen, her dress is mistakenly coloured green. Her dress changes back to purple when she is shown again. (00:12:40)
Helga's Boyfriend / Crush on Teacher - S2-E14
Other mistake: Helga's Boyfriend: Near the start, Helga thinks of somebody to make Arnold jealous, then it cuts to a shot of Stinky in the cafeteria. In this shot, there are two men meant to be standing behind the counter in the background, but their bodies are actually embedded through the middle of it. (00:01:46)
Helga's Boyfriend / Crush on Teacher - S2-E14
Other mistake: Helga's Boyfriend: When Arnold and Stinky are on the swing set, Stinky tells Arnold that he is crazy in love with a girl. Arnold asks him who the girl is, and in the next shot of Stinky, his head disconnects from his body three times. (00:06:40)
Helga's Boyfriend / Crush on Teacher - S2-E14
Other mistake: Helga's Boyfriend: At the Stake Time roller rink, when Helga and Stinky are spinning around like a tornado, many of the kids are standing in mid-air. Brainy and Park are also standing in mid-air in an earlier shot where Helga tells Stinky, "Let's go!" (00:03:58)
Helga's Boyfriend / Crush on Teacher - S2-E14
Other mistake: Helga's Boyfriend: When Helga asks Stinky to be her boyfriend, as Stinky says, "Well, could you make it a Mr. Nutty candy bar?", you can see another Stinky sitting at the table at the far right. (00:02:16)
Helga's Boyfriend / Crush on Teacher - S2-E14
Other mistake: Helga's Boyfriend: At the start, Helga starts to profess her love for Arnold in the movie theatre. In this shot, Arnold's hair has four spikes on the right and three spikes on the left. Arnold's hair is supposed to have five spikes on the right and four spikes on the left. Also, just as Helga says, "your every action," Arnold's left side hair changes to four spikes mid-shot. (00:01:06)
Hall Monitor / Harold's Bar Mitzvah - S2-E15
Other mistake: Hall Monitor: After all the kids run into Phoebe, when Sid runs past her and her hat comes off, we can see that the hair above her scrunchie is yellow instead of black as it should be. (00:03:44)
Abner Come Home / The Sewer King - S1-E16
Other mistake: Abner Come Home: When Arnold wakes up and starts looking for Abner, as he walks into the kitchen, his hair clips through the doorway. (00:04:14)
Abner Come Home / The Sewer King - S1-E16
Other mistake: Abner Come Home: In Abner's flashback, Grandpa shows him how to get in and out of the door. We see grass outside the door, but what we should actually see are the stairs that lead to the door, not grass. (00:03:08)
Love and Cheese / Weighing Harold - S4-E7
Other mistake: Love and Cheese: When Arnold and Lila's boat is stuck at the edge of the waterfall, as Lila says, "Arnold, what will we do?" a piece of the boat next to Lila is incorrectly coloured like the background and the seat, so it looks like a hole in the boat. (00:10:14)
Hall Monitor / Harold's Bar Mitzvah - S2-E15
Other mistake: Hall Monitor: When Helga is deciding on the new hall monitor and it shows a shot facing the crowd, many kids sitting in the back are faceless. (00:02:20)
Hall Monitor / Harold's Bar Mitzvah - S2-E15
Other mistake: Hall Monitor: When Helga chooses Phoebe to be the new hall monitor, as Phoebe is walking to the stage, Iggy's hair is incorrectly coloured black instead of brown. (00:02:30)
Timberly Loves Arnold / Eugene, Eugene! - S5-E22
Other mistake: Eugene, Eugene!: When the kids sing "Sunny Side Up" at the end, as they sing, "Who cares if the day is foggy and gray?", the crown of Lila's hat disappears for one frame. (00:22:46)
Runaway Float / Partners - S2-E20
Other mistake: Runaway Float: When Bob asks the kids who wants to ride on the float, Arnold says to them it's great the way it is. In one shot, Iggy's hair is incorrectly coloured black instead of brown. (00:05:52)
Back to School / Egg Story - S4-E8
Other mistake: Back to School: After Phil completes his long division on the chalkboard and it cuts to a shot of Arnold, the shot is flipped, with Arnold's hair being the most notable giveaway. (00:04:43)
Helga's Locket / Sid and Germs - S5-E7
Other mistake: Helga's Locket: When Arnold and Grandpa are in the basement and Arnold says that they need the Shiska-Bob Stick, a small portion of the brown paper bag on the workbench is missing. (00:10:06)
Helga's Locket / Sid and Germs - S5-E7
Other mistake: Helga's Locket: When Helga first sneaks into the boarding house, Grandpa is shown trying to open the locket. When the camera pans over to Helga and stops, a small part of Grandpa is still visible; then he disappears mid-shot. (00:03:25)
Simmons' Documentary / Big Bob's Crisis - S5-E20
Other mistake: Big Bob's Crisis: When Bob faints at the restaurant and first appears as an angel, his eyes become the same colour as his skin twice. (00:13:58)
Timberly Loves Arnold / Eugene, Eugene! - S5-E22
Other mistake: Timberly Loves Arnold: When Harold is leaving the field after the baseball game, he vanishes while on screen without fully leaving the frame. (00:06:15)
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.