Revealing mistake: After Arwin scores the winning spare, Esteban and Mr Moseby lift him onto their shoulders. When they drop him there is obviously a large pad to catch him as you can see his legs stick up in the air between Maddie and Esteban. It is even more noticeable as he bounces up and down on the pad a bit.
Revealing mistake: When Arwin gets both the seven and the ten bowling pins causing Team Tipton to win, if you look very closely at the close up of when he hits them, you can see that someone is controlling the pin with something to make it hit the other one, because right before he hits it, you can see the pin jerk a little before anything touches it. Pretty obvious.
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.