Continuity mistake: When London and Mr. Moseby crash through the wall with a car, there's a white betong-block on top of the car, but a few shots later, it's turned itself around a little bit more to the middle. (00:12:35)
Visible crew/equipment: At the beginning, you can see studio lights reflected on Zack's guitar during the song "Schools Out". (00:00:55)
It's a Mad, Mad, Mad Hotel - S1-E12
Visible crew/equipment: When Arwin tells about his super strange glue, there's a studio light reflected in his glasses.
Visible crew/equipment: In the episode "Hotel Inspector," when London comes to tell Maddie about something, she says, "Dogs playing poker," and you can see studio lights on her sunglasses.
Continuity mistake: Zack and Cody's mom takes away the rats to Zack and Cody's bedroom. She goes behind the sofa, but in the next shot of the door, she's not to be seen anymore.
Visible crew/equipment: When Cody tells his mother how much he hated Zack, you can see the reflection of a studio light on the chair he is sitting on. (00:08:45)
Grounded on the 23rd Floor - S1-E5
Continuity mistake: Zack and Cody skate in the lobby and push a guy with a golf bag. It falls and spills the golf clubs onto the floor. A big guy trips on it and falls over Mr. Moseby, and after that the golf bag is not seen. (00:00:35)
Grounded on the 23rd Floor - S1-E5
Continuity mistake: In the episode 1x05 "Grounded on the 23rd Floor", Zack and Cody give all the candy for a camera. They put it all on the table, but when they get the camera, all the candy is gone. (00:08:00)
Visible crew/equipment: In the episode "Hotel Inspector", when the man with glasses comes in, you can see a light on his glasses that looks like a studio light. (00:18:30)
Grounded on the 23rd Floor - S1-E5
Continuity mistake: In the episode 1:05 "Grounded on the 23rd Floor", when Cody falls from the porthole, two glasses are on the floor, but later when they run after the papparazi, the glasses are gone. (00:15:45 - 00:18:25)
Grounded on the 23rd Floor - S1-E5
Visible crew/equipment: At the beginning of the episode 1:05 "Grounded on the 23rd Floor", when the big guy is falling over Mr. Moseby, you can see studio lights in his eyes when he yells "mamma". (00:00:40)
Continuity mistake: In the episode "Commerical Break," they start to film the commerical for the first time without London and with Mr. Mosby. London gets upset and sits down on a couch where they are shooting the commerical. She and the couch leave the shot for a second, and London is gone when they show the couch again.
Visible crew/equipment: In the episode 1.09 "Band in Boston", Arwin comes in with his "Saxoblower." He puts on sunglasses and you can easily see studio lights reflected in them. (00:12:55)
The Prince & The Plunger - S1-E6
Visible crew/equipment: Zack and Cody go to Arwin the first time and get donuts, but when Arwin says it's going to be a steak next time you can see the studio lights reflected in his glasses. (00:09:15)
Continuity mistake: In "Boston Holiday", Zack is talking to Mr. Moseby in the hallway. In a wide shot, behind Zack, you can see that the elevator door is closed. But when it cuts to a close-up, the door is suddenly open. There wasn't enough time between shots for the door to open.
Continuity mistake: Zack puts Cody's whistle in their mate's pudding, but when Cody says, "You got pudding on my whistle" and the mate says, "And you got whistle on my pudding," the whistle is gone from the pudding, between shots. Then, a few shots later, we see that Cody is holding the pudding drenched whistle. (00:08:00)
Grounded on the 23rd Floor - S1-E5
Visible crew/equipment: When Maddie gives Zack and Cody the "US People" magazine, you can see a studio light on it. (00:07:00)
Grounded on the 23rd Floor - S1-E5
Visible crew/equipment: At the beginning of the episode 1x05 "Grounded on the 23rd Floor" after the big guy falls over Mr. Moseby and everybody looks at them, look at Zack and Cody's helmets: you can see a studio light. (00:00:50)
Grounded on the 23rd Floor - S1-E5
Visible crew/equipment: At the beginning of the episode 1x05 "Grounded on the 23rd Floor", when we hear the classic song, if you look closely at the door windows, you can see a studio light. (00:00:10)
It's a Mad, Mad, Mad Hotel - S1-E12
Other mistake: In the episode "Its a Mad, Mad, Mad Hotel", when Esteban has his dream of becoming rich and London becoming his "Bell Girl", he calls the hotel "Hotel Esteban, but London's uniform still has a "T." When it cuts there is an "E" on the front desk.
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.