Revealing mistake: This is very obvious. When Zack comes back with the bike and gives it to Cody, and it breaks apart, you can definitely see at one point the string/wire that holds it together.
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.
The Suite Smell of Excess - S2-E15
Revealing mistake: When Arwin tries to use the P.U., when the door is closed, you can plainly see a reflection of Cole Sprouse waiting offstage.
Revealing mistake: When Arwin admits he cheated, on one of Arwin's lines, the guy behind Mom (the guy with the clipboard) silently mouths along with that line.
Revealing mistake: The whole time London is on Maddie's back and is dropped off, the stunt doubles' faces are noticeable.
Revealing mistake: At the end of the episode, when Moseby and the rest do the dance number, London falls from the stairs onto the floor at the bottom, near the candy counter, when she hits it, you can see the corner of the blue crash mat come up.
The Suite Life Goes Hollywood (2) - S2-E37
Revealing mistake: When Maddie and London are pulled off the cart by the horse, you can tell they use dummies. Look at the way their bodies move.
The Suite Life Goes Hollywood (2) - S2-E37
Revealing mistake: In the scene where London and Maddie fight and fall off the the saloon balcony, you can see that they are stunt doubles.
The Suite Life Goes Hollywood (2) - S2-E37
Revealing mistake: In the scene where Maddie and London come out of the saloon scratching and clawing, you can see that they're played by stunt doubles; the stunt double playing Maddie doesn't have any bangs, whereas Maddie has bangs.
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.