When the Brain is shown on pages 2-4 at school, his complexion is light brown. While at Arthur's birthday party on pages 12-13, his complexion is fair skin.
On Page 13 during the birthday celebration, the stack of plates on the bottom central-right appear to be set down on the table. If the player clicks on the stack of plates, Sue Ellen pops up with the stack of plates above her head and laughs for about a couple seconds before retracting. She would have needed to pick up the stack of plates before putting them on her head than in the position they appear in.
Arthur's note to Muffy placed in her mailbox visibly is not put in an envelope. It should be put in an envelope and remain a secret before the recipient should come to retrieve her letter, and open it before reading the letter inside.
Arthur writes in a letter to his friend (and birthday twin) Muffy that his present for her is too big to carry by himself and that she should come to his house to pick it up. If it's too heavy for him to carry alone, he should at least volunteer to help her out considering they're both kids as it would also be too heavy for her to carry by herself.