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)
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.
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.
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.