Love and Cheese / Weighing Harold - S4-E7
Continuity mistake: Love and Cheese: When Arnold and Lila's boat starts sinking, it starts to move backwards and hits a huge rock at the edge of the waterfall. When Lila almost falls and Arnold holds her up, the rock is gone. It reappears in the following shots. (00:10:07 - 00:10:40)
Grandpa's Sister / Synchronized Swimming - S4-E12
Continuity mistake: Synchronized Swimming: In the pool scene at the beginning, we see a close-up shot of Connie and Maria. In the next shot, they are nowhere to be found. The three kids surrounding the pool in the beginning also disappear. (This happens because the opening shots are reused from the episode "Sixth Grade Girls.") (00:12:01)
Helga's Parrot / Chocolate Turtles - S4-E6
Audio problem: Helga's Parrot: After Arnold accuses Lila of flicking his ear, as he says, "Well, somebody flicked it," his mouth only says, "Well." It never says, "somebody flicked it." (00:08:38)
Weird Cousin / Baby Oskar - S4-E11
Continuity mistake: In "Baby Oskar" the apartment number on the door to Oskar and Suzie's apartment appears and disappears multiple times throughout the episode.
Dinner for Four / Phoebe Skips - S4-E5
Other mistake: When Helga and Phoebe leave the girls' bathroom together near the end, Helga pushes the door outward, but the hinges are on the inside, so it should only open inward.
It Girl / Deconstructing Arnold - S4-E9
Continuity mistake: When Eugene breaks into the area with the dogs, you see the pedals of his bike tossed into the air, but the pedals fell off on the other side of the street.
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).
Love and Cheese / Weighing Harold - S4-E7
Character mistake: Love and Cheese: When Arnold and Lila are sitting at the table at the Cheese Festival, Arnold asks Lila, "Do you want to ride on the Tilt-To-Whirl?" However, the ride is actually called "Built-To-Hurl." (00:06:59)
Love and Cheese / Weighing Harold - S4-E7
Plot hole: Love and Cheese: Arnold and Lila go to the Cheese Fair together in this episode. However, the episode "Operation: Ruthless" also takes place at the Cheese Fair, and it is stated in that episode that the Cheese Fair is an annual event. It can't be that this episode takes place a year later, as the series takes place during the same fourth-grade school year.
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)
Love and Cheese / Weighing Harold - S4-E7
Continuity mistake: Love and Cheese: When Arnold is trying to knock the bottles down and Lila gives him a flirty look, her collar turns green for one frame. (00:06:12)
Love and Cheese / Weighing Harold - S4-E7
Continuity mistake: Weighing Harold: When Arnold shows Harold his reflection in the water, the reflection of his tooth is not reversed, it is on the same side of his mouth. The second time his reflection is shown, it is reversed. (00:16:37)
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.