Acceptable Targets: Paris Hilton, as a means to subvert No Celebrities Were Harmed. They even mention Paris Hilton by name in several episodes. In an alternate universe she's president of the United States, and declares being fat illegal.
Characterization Marches On: Originally, London seemed to be set up as the Alpha Bitch, although she was quickly developed into being a well-meaning idiot. Likewise, Maddie was probably developed as sort of being a nicer foil to London, although she became much snarkier as the show went on to the point of her every line involving insulting everyone. This is lampshaded in one episode where they compete for a boy, with Maddie assuming he'll reject London for being a Rich Bitch....only for him in the end to point out the Maddie is "poor and mean."
"On Deck" seemed to go back and forth between London being mean and ignorantly insensitive.
Maddie explaining to London that a shoelace tip is called an aglet may have been a one-off joke, but if you take Ashley Tisdale's other role into consideration, which had an entire episode about aglets.
Another one, in the original series, Moseby was asked where the twins were going, he responds, "With any hope, The Bermuda Triangle." Guess where they head in the Sequel series.
The above mentioned Paris Hilton being president in an alternate universe. In 2008 Paris released a parody video for Funny Or Die where she said she planned to run for president.
Hollywood Homely: Ashley Tisdale, as Maddie, is supposed to be the homely one in comparison to London. It's a downplayed example as Zack has a Precocious Crush on her from the start. In the 9th episode "Band in Boston" there is this conversation.
Acting as parents for a baby doll for a school project.
After a heart-to-heart talk about her father, London tries to buy diamond jewelry for Maddie.
"Poor Little Rich Girl":
London: "You took me in when no one else would. You like me for richer or poorer, for better or worse, in sickness and in health-" Maddie: "We're not married, London."
"Time to strip and make the beds!"
London:(starts taking off her clothes) Okay, if that's what poor people do...
Maddie: Whoa there, pretty girl!
In Suite Life Goes Hollywood, the Hollywood studio got Dante Basco to play London in their Show Within a Show, with their excuse being that they wanted a romance between the candy counter girl and the owner's kid. London's response?
London: But I would never date Maddie! She's not my type! She's poor.
Retroactive Recognition: David Blue, who would go on to star as Eli Wallace in the SyFy series Stargate Universe appeared in the episode "Nurse Zack" as Dr. Chip Walters, the Tipton's young, inexperienced, and unprofessional staff physician.
Cole Sprouse (Cody) played Ben in Seasons 6-8 of Friends.
Seasonal Rot: A notable change in quality occurred halfway through the second season.
What Do You Mean, It's Not Symbolic?: The Christmas episode has a couple named Mary and Joseph who couldn't get a room in the Tipton but are stuck there thanks to the snow. Later Mary goes into labor and has a child in the elevator on Christmas eve. The cast Lampshades how similar it is to the birth of Jesus.
Woody/Addison is very popular, due to being entirely plausible, no objections against it, and being almost Canon, if Woody just had the guts to ask her out.
Foe Yay: Zack and Bailey in "Seven Seas News" akin to the movie its parody.
"Funny Aneurysm" Moment: In "The Defiant Ones," Cody tells Bailey that he lied to Miss Tutweiller, saying that he was taking care of Bailey's sprained ankle so he couldn't do his project, because he didn't want to besmirch his perfect track record and risk not getting into Yale with Bailey like the couple had planned, and instead ending up at Princeton. At the time, this is amusing, because one missed assignment isn't going to make that much of a difference, and it's not like Yale won't accept Cody, right? Fast forward to the finale, when Bailey gets into Yale, and Cody does not.
An author once wrote two stories about romances between Bailey and Jessica Ellis (who at the time was just one of two twins), then "Model Behavior" happened and Jessica and Bailey shared several Les Yay ridden scenes.
In another fanfic called 'Zoey 1803', Zack is turned into a girl named Zoey. Zack would later get a love interest played by a girl named Zoey.
In "My Oh Maya", Maya (said love interest played by Zoey) lists various drinks the smoothie bar sells. One of them is the Cherry Chiller. It was easy to miss at the time but fastforward to 2012 when McDonald's released a smoothie with a similar name.
In Maddie on Deck the episodes antagonist is a childish young prince who turns out to be a pushover, and abuses his power to get what he wants. The princes named Jeffery. Replace the first E in his name with an O and you'd think this was a Game of ThronesShout-Out if it weren't for the fact Game of Thrones aired a few years after Maddie on Deck.
Hollywood Homely: Zigzagged with Bailey. She's portrayed by Debby Ryan, who is not even remotely unattractive, but she's constantly belittled for her looks, especially by London, but at other times, is praised for her looks, particularly by her Love Interest Cody, but even by Zack, who once called her "smokin'."
Bob and Zack seemed to have a deeper relationship than just friends. Zack seemed very hurt that he had become mushy with Barbara. Of course, it was only played for laughs.
The recent "Beauty and the Fleeced" episode combined this with a Stupid Sexy Flanders incident where at the beginning Woody (sans glasses) is apparently describing an attractive girl before Zack points out that Woody just mistook Cody for a girl.
This could also be seen as a shot at Cody's masculinity
"Ala-ka-Scram" definitely makes an argument for Zack and Woody, especially during their makeup scene.
Although only for one episode Bailey and Jessica have a ton of this with some fabricated Foe Yay (to fool Cody) included.
Replacement Scrappy: Maya was briefly this during the beginning of Season 3, due to viewers thinking she replaced Bailey or Marcus. Speaking of which, Bailey herself was essentially this for Maddie at first (not help by the fact that she starts out as a complete Mary Sue).
Replacement Scrappy Series: Many people thought this when On Deck replaced the original.
Seasonal Rot: From mid-Season 2 into the first few episodes of Season 3, wherein most characters' Flanderization came to a peak, and watching the show became something akin to rubbernecking at the train wreck. Once Season 3 got off to a roll, the characters began to recover their charm and depth.
Strangled by the Red String: As mentioned above, Cody and Bailey border on this. While they do have similar interests and personalities, in nearly every episode they are a couple, they constantly fight and someone has to go out of their way to remind them what a great couple they are so they would get back together. If you need to constantly remind a couple how great they are, then they probably are not doing so good anyway. Also, when they break up, they keep mentioning the breakup, bordering on As You Know.
Fanon Discontinuity: It's much more enjoyable if you think of it as an adaptation of the series and don't try to fit it into the timeline.
Tear Jerker: After Zack runs a submarine into the seawall in an attempt to impress a cute science assistant, leading Cody to do the same trying to stop him, Cody gets fired and kicked out of the program. That's not the real tearjerker—that comes in a subsequent scene when Cody tells Zack he hates him and states that despite being twins, they are not brothers.