Revealing mistake: The flags on Tipton hotel you see almost every episode never move; they're animated.
Revealing mistake: The top of the Tipton Hotel is animated. You can tell because in the opening of any episode showing the top of the hotel, there should be 3 visible peaks. You can see 2 peaks and if you look where the middle peak should be, you can see through the top of the hotel to the sky on the other side.
Visible crew/equipment: Sometimes when they're at the pool and someone has sunglasses on, you can see the reflection of a studio light.
Other mistake: The characters that are supposed to be native to Boston, Massachusetts, i.e. Maddie, Maddie's parents and Red Sox fans, don't ever actually speak with the iconic Boston accent.
Revealing mistake: London's phone screen is visible, and it is shown that it is not a real digital phone.
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.