The episode where they put on High School Musical might as well be called "Hey, Did You Know Ashley Tisdale Also Plays Sharpay?" Gets bonus points for Tisdale performing Lucas Grabeel's parts in "Bop To The Top".
Dawson Casting: Not so noticeable in the beginning when 17-year old Brenda Song was playing 15-year old London Tipton, but by the end of Suite Life On Deck in 2011, London was a high school senior and Brenda was 23. Ashley Tisdale played 15-year old Maddie in 2005 when she was 20, but it's unclear if Maddie aged as slowly as London or if it just took London a few tries to make it through high school. (though On Deck makes it clear.) In any case, Ashley was 23 when "Suite Life of Zack and Cody" ended. It was worse for minor character Mary Margaret, who was the same age as London and Maddie, but played by Monique Coleman, who is five years older than Ashley and eight years older than Brenda. Averted completely with Zack and Cody themselves, who were literally the exact same age as their characters, starting the series off 12 and graduating at the end of the series at 17-18.
It's pretty clear that London has been held back. In "On Deck", Tutweiller mentioned that London had been voted Prom Queen her last 3 senior years, confirming she had been held back. Plus the twins, who are three years younger, graduated with her. So in the final season of Zach and Cody, London and Maddie are seniors and Zach and Cody are freshmen. At the start of "On Deck", London is still a senior and Zach and Cody are sophomores. So London doesn't really fit this trope too much as London would be 21 while Brenda was 23.
Fake Mixed Race: London Tipton's mother is revealed to be Thai and father is revealed to be white. London is played by Brenda Song, who is both Hmong and Thai.
Fake Nationality: Foreigners are generally played by American actors. This was also once Double Subverted — they visit England, and the guest star of the week is Charles Shaughnessy, who actually is British — but by the end of the episode, the character has been revealed to be Belgian. Go figure.
In one episode, it's revealed that London is at least part Thai. Her actress, Brenda Song, is of Laos descent.
Fake Russian: The episode "Das Boots" had Sasha Matryoshka, a Russian junior chess champion played by Cody Kennedy, who's of Russian ancestry, but otherwise American as apple pie.
Promoted Fangirl: Debby Ryan had a few acting credits but was otherwise just a normal teenager and huge fan of The Suite Life of Zack and Cody before becoming Bailey.
Retroactive Recognition: The magician from the episode "Alaka-scram!" is Justin "Kredible" Willman, a real magician who has appeared in several TV specials and currently hosts Disney Channel's revival of Win, Lose or Draw
Arguably a borderline example since she did have her name on the opening credits every episode, but Debby Ryan has since become a major star for Disney Channel in JESSIE.
Also, the Prince of Lichtenstamp in the same episode where Ashley Tisdale showed up later became Joshua Matthews in his teens.
And Doc Shaw would go on to be one-half of Pair of Kings (hence his Season 3 departure).
What Could Have Been: Dylan Sprouse said he and Cole wanted to have creative control of the show for a potential 4th season, as well as a pay increase and producer credit. They pitched an idea where Zack and Cody return to the Tipton while the two attend college. They would also become friends/mentors to a boy who lives with his single father in the hotel, showing him all the fun one could have at the Tipton. Once the season ended, the boy and his father would get their own show via a spin-off. Disney vetoed their pitch idea and gave their own idea which, according to Sprouse, was the same as theirs but with a few twists (and did not increase their pay and give credit). The potential show would have involved Zack going to work in Miami with Cody and would've had Selena Gomez attached. There were rumors saying that the executives liked Debby's idea more than the Sprouses' (Which led to the aforementioned Jessie), and the rest is history.