Continuity mistake: When one of the older twins in the wheelchair throws the apple and his twin catches it, we see that nobody is standing on the stairs in front of the main entrance. Though in the previous and following shots Moseby is actually standing on the second step.
Continuity mistake: When Zack trips on the slick floor, his red backpack ends up behind Cody, but in shots facing the kitchen the backpack is across the room, near the kitchen cart.
Answer: Keep in mind, Muriel wasn't very popular. So the writers/directors must have thought it'd be a good idea to let go of the character. No one seems to be complaining.