Dinner for Four / Phoebe Skips - S4-E5
Other mistake: Phoebe Skips: When Lila tells Helga, "I just didn't feel right about destroying Rhonda's personal property," she blinks, and her right eyelid is not colored in. (00:16:19)
The Beeper Queen / Oskar Can't Read? - S4-E4
Continuity mistake: Oskar Can't Read?: When Oskar comes home at the end, Ernie and Mr. Hyunh are standing on Arnold's left. As they walk inside the house, they are on Arnold's right. (00:22:36)
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)
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)
Love and Cheese / Weighing Harold - S4-E7
Other mistake: Love and Cheese: When Arnold accidentally kisses Gerald's hand on the bus, his left foot is missing. (00:02:00)
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.