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.
Audio problem: When Cody orders the Number 8, someone says, "We got a Number 8". But no one's mouth moves.
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)
Visible crew/equipment: Sometimes when they're at the pool and someone has sunglasses on, you can see the reflection of a studio light.
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)
Continuity mistake: In the scene where they are having the finals, the things on the desk change a few times.
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.
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.
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.
Continuity mistake: When Zack and Cody finally come to the stage, Cody has a sock over his sweater and it's still there when they start singing. But in an instant the sock is gone. Cody could not have taken it off, because he was too busy playing piano.
Other mistake: When London is reading her book to the little children, the small girl wearing pink with a ribbon in her hair is miming what London is saying, just before London actually says it.
Visible crew/equipment: When Chef Pablo shows them the greatest food, you can see the studio light and camera reflecting off the domes.
Visible crew/equipment: In London's suite, when Arwin opens the door to the porch where the hawk is nesting, the reflection of a crewmember wearing a white shirt is visible on the door's glass panes.
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.
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.
Visible crew/equipment: Episode 1x09 "Band in Boston", the reflection of studio lights are visible on the drums almost every time they are seen during this episode.
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.
Grounded on the 23rd Floor - S1-E5
Continuity mistake: When Carey opens the suite door to find Moseby with the hotel bill, then when the room service arrives and in the following shots, the twins' backpacks' straps are on, then off, then on, then off and so forth.
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.