If you ever watch Hey Arnold, you probably wouldn't have noticed this. In the opening credits when Helga and Arnold are walking towards each other, when it shows both groups there are 6 boys, 6 girls, but when it is up close there are only 4 of each! they cut of the people on then ends.
I also, have noticed this every single time, no lie, it's not a slip-up but they just did a close-up. That's all, and I'm pretty sure they were just looking at the main characters.
Actually, the main slip-up person, first, there is 6, then there is 4, it STAYS 4 when the "Camera" shows the people sticking their head out....i think