Visible crew/equipment: When Zack, Cody, Tapeworm and Max are singing "School's Out" on stage, you can see the shadow of the camera crane.
Visible crew/equipment: Season 1, episode 9 'Band in Boston': After Zack dips Cody's whistle in the pudding, when Zack rips up Cody's music have a look at the floor where you will see the actors' tape marks.
Continuity mistake: Season 1, episode 9 'Band in Boston': When Cody and Zack are arguing over Zack's look (vs Cody's lyrics), the black cord (with whistle) goes from under Cody's shirt collar, to over, back to under, between shots.
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)
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.
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.
Continuity mistake: Season 1, episode 16 'Big Hair & Baseball': In the lobby, when Cody comes to the defense of Moseby, Zack is holding two ice cream cones and as he continues eating, more of the ice cream appears between shots.
Continuity mistake: Season 1, episode 19 'Ghost of 613': When London finds her gold purse in suite 613, she places it on her left arm just as the noise at the door scares her, but when Cody tries to jump into her arms the purse is on her right arm.
Christmas at the Tipton - S1-E21
Continuity mistake: Season 1, episode 21 'Christmas at the Tipton': When Arwin leaves to fire up "Old Betsy", he trips over the couch and when he breaks the table its broken pieces differ between shots.
Continuity mistake: Season 1, episode 22, 'Kisses and Basketball': The twins' mom is sitting in the back and she has her hands down. When Arwin says "Let's kick some" it cuts to their mom and she has her hand on her neck, it then cuts back to the basketball team and she has her hands down by her lap again.
Visible crew/equipment: Season 1, episode 24 'Crushed': When Esteban holds the tray with the dogs' food, the studio's numerous overhead set lights are reflected on the surface of the large, silver, dome lid.
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.
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.
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.