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)
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.
Dinner for Four / Phoebe Skips - S4-E5
Other mistake: Dinner for Four: When Arnold and Lila are talking to each other in the beginning, Lila says that she hopes to become a famous ballerina one day if she practices hard enough. In this shot, the stripe on her shirt is incorrectly layered over her gold button. (00:01:26)
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)
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.
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)
Love and Cheese / Weighing Harold - S4-E7
Other mistake: Love and Cheese: When Arnold and Lila grab each other's arms on the boat, part of Arnold's left arm becomes invisible. (00:10:18)
Love and Cheese / Weighing Harold - S4-E7
Other mistake: Love and Cheese: At the end, when Arnold says to Lila that he will walk her home, they both start walking to the right, then Lila vanishes while on screen. She never leaves the frame. (00:11:42)
Back to School / Egg Story - S4-E8
Other mistake: Egg Story: When Arnold tells Helga, "I know you don't like me, and frankly, you're not my favorite person either," his mouth disappears twice. (00:14:51)
Grandpa's Sister / Synchronized Swimming - S4-E12
Other mistake: Synchronized Swimming: When the boys take off their jackets during the swimming tournament, Arnold's jacket is floating in mid-air, and the left side of Eugene's jacket is also in mid-air. (00:20:45)
Grandpa's Sister / Synchronized Swimming - S4-E12
Other mistake: Synchronized Swimming: Right before the boys jump into the pool during the swimming tournament, Gerald's right eye seems to have lost its color. Right when Sid jumps into the pool, his eye is white again. (00:20:52)
Grandpa's Sister / Synchronized Swimming - S4-E12
Other mistake: Synchronized Swimming: During the swimming tournament, when the boys jump in the pool and it cuts to the crowd, Lila's dress and bows are incorrectly coloured blue, and they stay that colour for the rest of the episode. (00:21:04)
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.