The Big Scoop / Harold's Kitty - S2-E3
Corrected entry: Arnold's phone number is "555-123", but all phone numbers in the USA have seven digits.
Helga's Love Potion / Gerald's Secret - S2-E2
Corrected entry: When Helga goes into her closet, she has quite a few different dresses, but the one she always wears isn't there.
Correction: Of course not, she's wearing it.
Mud Bowl / Gerald Moves Out - S2-E9
Corrected entry: How did the fifth graders manage to forget how to score touchdowns, considering that they scored six or seven in the first half and none of Arnold's special plays were for when his team was on defense?
Correction: They scored much more than that before Arnold's special plays were done. They got 42. They didn't score any when Arnold's special plays were done because they weren't used to them, and they were pretty tricky.
Part-Time Friends / Bio Square - S2-E19
Corrected entry: Towards the end of "Bio Square," Arnold and Helga manage to escape drowning in the titular Square (from a haywire sink leak) by jumping onto her inflatable mattress. The two ants from an experiment by Arnold manage to escape, too, by jumping off a table and onto a floating, half-eaten potato. However, at the end of the episode, when Arnold and Helga are describing their adventure before their science class, Arnold says that the ants "climbed onto the leaf." Knowing how sharp Arnold is, it's unlikely that he mistook the potato for a leaf. This is a simple script error.
Correction: Sharpness aside, Arnold is prone to making errors or mistakes. The two were about to drown and the seriousness of the situation might have mistaken him. Also at that point I don't believe there was anything to prove if he even knew the ants made it to safety so he might have believed they climbed onto a leaf. In any circumstance its at best a character mistake.
Correction: This is true but tv shows, movies, ect. have license to ignore this fact because they could get sued were they to use a real 7 digit number, even if it was accidental, without permission. So this mistake is not valid as the makers have to do this.