Darker and Edgier: "City of Thieves" is this compared to the rest of season one. As is "What Have You Done?"
"Ocean of Fear" is all about facing the physical embodiment of fear, which makes for a much darker episode. In addition, Pat McHale's quote at the end of the show gives one pause. This is also one of the few times we get serious evidence of Ooo's post-apocalyptic feel.
The second season, especially the finale.
Princess Bubblegum: If you fail... he'll kill everyone.
"Jake vs. Me-mow" features an assassin named Me-mow, a tiny cat who stabs someone to deliver a death threat to Wildberry Princess, and threatens to inject Jake with a deadly poison if he doesn't kill Wildberry Princess himself. Jake just grows his liver and detoxifies himself.
The Ice King's origin in the episode "Holly Jolly Secrets Part II". It was probably the darkest back story in Adventure Time and on Cartoon Network - He was once a human named Simon Petrikov, driven insane by his arcane crown.
The fruit witch scene in "Dad's Dungeon", which happens to feature blood and bones. Specifically the blood and bones of anyone who eats one of the small apples, as those turn them into giant apples on the outside. Finn had a pretty good idea of what would happen if he ate the food, but was going to anyway due to feeling like everyone he cared about turned against him. Jake saves him by swiping the apple and forcing one of the three witches to bite it, causing her to transform and the other two to eat her.
Marceline once inhabited a realm known as the Nightosphere, a type of Hell where demonic creatures petition Hunson Aberdeer for boons. Due to his tendency to wear a mystic amulet of chaotic evil, he usually grants them in grotesque ways.
Marceline whilst wearing amulet: You sure dude? Don't you want 'abs'?
Half Monster Demon: Get in my flippin' belly and I will transport you. My insides are habitable. You will not be digested.
Princess Bubblegum: I'm not gonna live forever, Finn. I would if I could. But modern science just isn't there yet.
Also, Goliad's monologue about the bee.
Goliad: Bee doesn't care about flower. Bee will hurt flower to get what it wants. Bee is stronger than flower. *crushes bee under its palm*
In "Princess Cookie", Finn and Jake have to take care of a hostage situation, orchestrated by a cookie named Baby Snaps. Baby Snaps escapes with Jake only to have this sequence when the pursuit gets close: (He gets better.)
Baby Snaps:I'm sorry, Jake. It's over. *zoom to close, falls backward over cliff, smashes into pieces*
The series as a whole has become much darker. While theres still loony silliness all around, there are way more melancholic undertones to the show than before. Compare a recent episode to one of the earlier ones which featured candy parties and thawed out businessman.
"Lady & Peebles", in which Lady Rainicorn is revealed to be pregnant in the last seconds and set in the organic lair of a conscious heart often tombed in the Ice King, mostly.
In that same episode, Princess Bubblegum challenges Ricardio to hand-to-hand combat, before expertly dismantling him.
Princess Bubblegum: You think we're intellectual equals?! It only took me seconds to get you off your guard! And this body you designed is self-congratulatory garbage! See, I know a thing or two about building a body out of biomass, *Detaching leg* and you don't... leave your heart exposed!
Ricardio: I just wanted to impress you!
Princess Bubblegum: You didn't. *Pounds Ricardio's face with his own leg* Get out of my sight.
"The Lich": It's bad enough to see the Lich wish for the extinction of life, yet when Finn wishes for the Lich not to exist, it has unforeseen consequences. Mushroom War's radiation never happens. Add in the fact that the universe's 'Jake' ends up turning into the Lich when Finn finds the crown of the Ice King, and you've got one seriously messed up episode. It gets better, though.
In season 6's "Breezy," Finn explores the darkness of lust to search for a warm feeling he's lost. It's exactly what it sounds like.
Dark and Troubled Past: In the episode "The Apple Thief", Jake mentions that he stole purses and hawked stolen bicycles; which could explain his actions in "City of Thieves".
Baby Snaps was an orphan who had his dreams of being a princess created and ruined by Princess Bubblegum reading to the orphanage, then laughing at his idea that he could become a princess to help people like her.
Most of Marceline's thousand-years-long existence can be described as this. Her tragic backstory even ties in with Ice King's.
Darkest Hour: Finn goes through this for a short time in "Freak City"... Complete with awesome music!
Deadly Prank: Played with in "Heat Signature"; Marceline's ghost friends actually wanted Finn and Jake to die. They and Marceline had tricked Finn and Jake into thinking they were vampires, whose powers include flight. The extended prank culminated in the ghosts teaching them how to fly by knocking them off a tower (Marceline, drawing the line at actual harm, catches them). The ghosts did it because it would have been funny.
Jake: "I want to carve my face on the moon... so the moon will look like my face!"
Ice King: A leg cramp! In my leg!
Depending on the Artist: Each episode (especially in the second season onward) has slightly different character appearances due to the art style of the storyboarders of that episode.
Depending on the Writer: An in-universe example would be the Fionna and Cake fan-fiction. Each time a Fionna and Cake story is presented, including the comic, it's told by a different person, and the characters reflect that person's personality.
Jake: Nah... That could never happen. Finn: You didn't think this through enough. IT COULD HAPPEN!
Did They or Didn't They?: In "Breezy" Finn pecks Lumpy Space Princess on the lips, which makes her tell him that she doesn't want to be stuck in the "kiddie pool"; she wants to go to the "deep end" instead. There is a Fade to Black, and the next scene shows Finn and LSP lying down, separated.
Did You Just Punch Out Cthulhu?: In a misguided if courageous example, Jake almost kills Marceline, someone who effortlessly defeated Finn and him in an early episode, all while dressed as a strawberry and fighting his monumental fear of vampires to save his friend.
Finn gets a turn when he defeats the Lord of Evil, Marceline's dad with two swords, a smiley face, bug milk, and some magic words, all while screaming to the top of his lungs.
When Marceline's dad tried to suck out Gunther's soul, the little guy slapped him in the face.
The legendary hero Billy casting down the Lich. Bonus points for it being said that he pummeled him into a tree where he imprisoned him in amber.
Finn and Ice King manage to defeat the Lich again when he gets free. Not as utterly Badass as pummeling the guy into a tree, but still.
In "Betty", the titular character walks up to Bellanoche, who has so far been winning effortlessly against an army of powerful wizards, and knocks her out cold with a single punch. This was most likely due to the fact that Bellanoche's power allows her to absorb magical energy, which means a Muggle like Betty would be unaffected.
Digital Piracy Is Evil: "Videomakers" is about Finn and Jake deciding to make their own movies for movie night, since (pre-apocalyptic) federal law prohibits unauthorized exhibitions of movies.
Disappeared Dad: Finn's biological dad. That is until the Season 5 finale where Billy tells Finn that his dad can be found at the Citadel.
Disaster Dominoes: Surprisingly averted in "Apple Wedding, despite all the pieces being there like Cinnamon Bun shaking up the bottle of apple cider and Finn trying to prevent LSP from crashing the wedding. The only thing that amounts to anything is Princess Bubblegum hijacking the King of Ooo's balloon.
Also Princess Bubblegum is often barefoot, however her feet are often covered up by her long dress. Though with recent episodes she is shown wearing shoes more often. In Glitch she is back to being barefooted.
Flame Princess is always barefoot. She gained shoes with her new outfit but is back to being barefoot with her more recent outfit.
Bufo: Now I release the elder Jeremy from his pledge. Razzamafoo!
Elder Jeremy: It's about time. *crumbles into dust*
Bufo: And you take his place for the rest of your natural life.
Finn: Awesome!!! ...WaitWHAT?
Downer Ending: The episode "I Remember You" ends with Marceline singing the song written by Simon with The Ice King, while sobbing after trying the whole second half of the episode to make the Ice King remember who he was. He doesn't, she just cries as they both sing, Ice King completely oblivious to the situation, and it cuts to a Happy Flashback of Marceline when she was a crying little girl alone just after the apocalypse. Simon finds her, wipes her tears, and gives her the teddy bear she loses in Memory of a Memory.
"Frost and Fire" in which Flame Princess breaks up with Finn.
In "Puhoy" Finn finds himself in a dream-like world made of pillows. He soon realizes he can't leave and get back to his world and the episodes shows him marry, have kids, and become an old man until he eventually dies with his entire pillow family at his bedside. He suddenly emerges from the pillow world at the same place, time and age he was at when he left. Finn mentions to Jake that he had a crazy dream, but is distracted, and when asked again later he has completely forgotten the experience.
"Escape from the Citadel" Finn and Jake hitch a ride on the frozen body of the Lich and arrive at the Citadel. They end up finding Finn's father, named Martin, and the Lich manages to escape and free the other criminals, including Finn's dad. However, Finn's father has no memories of Finn, or why he even abandoned him in the first place. He ends up completely using Finn and tries to leave him in the Citadel and escape with the other criminals, while Finn and Jake try to fend off the Lich. Finn ends up coming close to being killed by the Lich, but manages to use Guardian Blood on the Lich(which restores Flesh and heals wounds) which causes it to be reborn as a giant baby. Finn then tries to stop his father from leaving by trying to hold back the craft they try to sail away through a portal on, but is close to reaching his limit and you can really see the strain on his face as he tries so very hard to keep his father with him. Eventually, the Grass Sword completely engulfs his arm to try and help him hold it back, but Finn finally can't hold on and lets go, ripping off the Grass Sword from his body, which severs his arm in the process. As he falls down, a drop of Guardian Blood lands on his severed arm, which only grows a flower in its stump because there were no bones or anything left of his arm to heal, implying that he may have lost it for good. Jake(and Shelby) comfort Finn and return back to Ooo, and they leave the Baby Lich with Tree Trunks and Mr. Pig.
"Betty". Ice King is temporarily turned back into Simon, who opens a time portal to apologize to his girlfriend Betty before he dies. Betty instead leaps through the portal to help him, and successfully saves his life. However, Simon reverts back to the Ice King and completely forgets about Betty. She sadly leaves to find a way to cure Simon of his curse.
"Bad Timing" has LSP hooking up with Johnny, one of her old classmates. However, LSP gets paranoid that Johnny might be cheating on her with Princess Bubblegum, and inadvertently "kills" him by misusing a time bubble on him. Devastated, LSP has PB erase all her memories of Johnny to escape the pain, and reverts to her old Jerkass self. Meanwhile, it's revealed that Johnny has been transported to the "border world" and witnesses LSP erasing her own memories. Even though Johnny is still alive, PB has no way of knowing, much less finding a way to bring him back.
Drama Bomb: Played straight and called out by Lumpy Space Princess in "It Came from the Nightosphere" when Marceline's dad hears Marceline's song.
The BIGGEST drama bomb in Adventure Time history came with the Christmas special "Holly Jolly Secrets" when Finn and Jake view the Ice King's biggest secret, on video, when he himself is right there with them.
Season three in general has been one great big drama bomb.
Season four didn't skimp either.
The first three episodes from the Flame Princess arc, but especially "Burning Low". The fourth, "Ignition Point", was a lighthearted fetch quest/dungeon crawl.
The ending of "Lady & Peebles", without question, is one of the biggest the show has ever had.
Lady:(In English) I'm pregnant!
"I Remember You" has Marceline's discussion of the Ice King's 'song inspiration'. The 'inspiration'? In the city ruins immediately after the Mushroom War, Simon Petrikov (before he lost his mind and became the Ice King) found Marceline as a young girl, alone and crying. He became sort of a father figure to her.
Dramedy: Would you believe a cartoon like this has its dramatic moments? One example is Marceline's song in "It came from the Nightosphere". Starts out with her talking about her dad stealing her fries, then turns into heart-breaking ballad about how he neglected her.
The show gets even more dramatic in "What Was Missing" when Marceline sings a deep song saying that she's annoyed that Princess Bubblegum treats her in an estranged manner, and then implies that she (Marceline) wants to make up with her in some way.
Jake gets some of these moments in "No One Can Hear You," and there's also him nearly making Finn cry in "New Frontier."
The Ice King has now been made into an extremely tragic figure, but even before then, he had sad scenes.
It has been established that the Ice King is NOT a good singer.
Played with in regards to Lemonhope. He playes the harp very well, which Lemongrab hates. He tries to play the flute, but is awful at it, yet Lemongrab loves it.
Driven to Suicide: Happens to Finn after believing that his father didn't think much of him in "Dad's Dungeon". Fortunately Jake manages to snap him out of it.
Also happens to Baby-Snaps in "Princess Cookie."
Dropped a Bridge on Him: Root Beer Guy in Something Big. Actually, the manner of his death is perfectly respectable, but still very anticlimactic since it happens off-screen before the episode even starts.
Even more egregious is that when the Big Bad is defeated, all of the candy soldiers that were killed are resurrected except for Root Beer Guy, meaning he's explicitly been Killed Off for Real.
Dual Wielding: Finn uses two swords in "Slow Love" and "It Came from the Nightosphere".
Dungeon Punk: The world is full of technology, such as Lady Rainicorn's universal translator device and holographic computers, mixed with magical items like wands and freezing potions.
The Dulcinea Effect: Played with in both "Incedium" and "Hot to the Touch". At the end of "Incendium", Finn is fighting off a demon of fire which extinguishes itself by Princess Bubblegum's water rockets. Only when the demon is extinguished is it revealed to be Flame Princess. Though she slaps him in the face after he says he likes her, he remains adamant on stopping her destruction while not hurting her in "Hot to the Touch", which begins directly after the end of "Incendium".
Dying as Yourself: Simon considers letting himself die in "Betty" instead of becoming the Ice King again.
Easy Amnesia: In "Death in Bloom", Jake gets amnesia from drinking the water in the River of Forgetfulness. Finn casually inflicts this on himself in "The Creeps", apparently something he regularly does when he experiences something disturbing.
Happens again in "Return To the Nightosphere" where Finn and Jake find themselves in a prison somewhere in a nightmarish realm with no idea how they got there and a banana near Finn. The sequel, "Daddy's Little Monster", later reveals through Jake's cell phone that they were hanging out with Marceline in the Nightosphere, but an evil amulet given to her by her father who wanted her to continue family business caused her to turn into a chaotic evil Eldritch Abomination knocking out the duo and giving them amnesia.
Eat The Camera: The end of the first part of "Holly Jolly Secrets" where Jake is screaming while the camera zooms into his mouth.
Elemental Rock-Paper-Scissors: Lampshaded in "Heat Signature," when Finn and Jake are being attacked by some ghostly friends of Marceline after a prank gone wrong.
Marceline: Finn, Jake... I'm so sorry I got you into this. I should've never pranked you so perfectly. Finn: It's okay. You're here now to beat up those ghosts... and save us! Marceline: Oh no, vampires can't beat ghosts. It's like a rock-paper-scissors thing.
Eskimos Aren't Real: Flambo, a catlike creature made out of fire, insists that ninjas "ain't real for nothing no-how."
Eternal English: The businessmen's weird grammar in "Business Time" can be taken as an aversion of this trope.
"We love work for you!"
Even Evil Has Standards: The Ice King will kidnap unwilling princesses left and right and try to make one of them his bride, but he loses interest in Princess Bubblegum when she's aged down to 13. He's creepy, but he's not THAT creepy.
And yet, he writes fanfiction about the genderbent version of Finn falling in love with him...
Everyone Calls Him Barkeep: Most of the royalty in the show, excluding Marceline. Princess Bubblegum also might be an exception since Bubblegum is really her surname.
Everything's Better with Princesses: There are a lot of princesses in the land of Ooo. Princess Bubblegum, Hotdog Princess, Lumpy Space Princess, Muscle Princess, Slime Princess, and so on. Then there's Princess Princess Princess. Pen's official stance on the abundance of princesses is "WHY NOT. AMIRITE? LADIES? GUYS? RIGHT? AMIRITE?"
Except for Dr. Princess. Princess is just her surname.
Evil vs. Evil: The final scene of the Farmverse: Ice King!Finn vs. Lich!Jake
Exact Words: In "Mystery Dungeon", the Ice King throws the book containing his Fionna and Cake fanfiction into a magical fire since it is said that anything thrown into the fire can be brought to life. However, only the book itself is brought to life, much to Ice King's dismay.
Exactly What It Says on the Tin: No, not the title itself (Thought it would kind of fit). In "Mystery Train" Jake grabs a train schedule. It has "you got a schedule" written on it.
In "Jake Vs. Me-Mow", Me-Mow boasts that she has injected Jake with enough poison to kill a dog fifty times his size. Which prompts Jake to use his powers to grow his liver fifty-one times its normal size, curing the poison.
Eye Scream: Peppermint Butler suffers this when he gets Princess Bubblegum's spicy potion in his eye in "Too Young".
Eyedscreen: "Ricardio the Heart Guy", This trick shows up when Finn says that Ricardio is "up to something."
Face Doodling: Finn and Jake do this to the Ice King when they steal the gems from his crown while he is asleep in "The Lich".
Faking the Dead: Finn in "What Have You Done?" to make Ice King wail and save the citizens of the Candy Kingdom.
Finn, Jake, Beemo, LSP, and Cinnamon Bun do this in "Conquest of Cuteness" to make the Cute soldiers feel better about themselves. LSP exaggerates hers by pretending to fall in love, then fall out of love with one of the Cuties, and consequently have a fake heart attack... for laughs.
Marshall Lee pretends to die in "Bad Little Boy" for the purposes of playing with Fionna's head and trying to get her to admit she loves him.
Familiar Maja's crabbit (crow/rabbit), but he really wants to be a dancer.
Family-Unfriendly Aesop:In universe, the lesson the Magic Man is apparently trying to teach anyone who encounters him is "be as big a Jerk Ass as you possibly can to people you come across"
Magic Man (after causing Jake's death in "Sons of Mars"): "I win again, as always!"
Family-Unfriendly Death: In "No One Can Hear You", a deer's head and neck are crushed by two cinderblocks, and then his body gets flushed down a giant sewer pipe!
Ghost Princess died after Clarence stabbed her. Clarence died after stuffing himself with so much soft cheese that he straight-up exploded.
The third season is fond of this trope apparently. In "Dad's Dungeon", a fruit witch is eaten until nothing but her blood-covered bones is left. And that was almost Finn.
Finn: Can we just get back to the brain-eating, or whatever?
Dying in the Fight King's arena results in you being trapped there as his ghost gladiators forever. Finn managed to kill the Fight King, setting the ghosts free.
From "To Cut a Woman's Hair":
"Well, maybe you should help me get some princess hair, so maybe your friend won't be sucked into my bottomless bottom!"
The fate of Princess Bubblegum and the Candy People in the episode "No One Can Hear You".
In "Betty", Simon confides that he'd prefer to die than become the Ice King again, but does so anyway, on the off-chance that he can be saved...
Fearful Symmetry: The Crystal Guardian of the Crystal Gem Apple. Unlike some examples through, it's not an actual clone, but an imitator made out of hard, sharp crystal. When you punch its punch, you're the one getting hurt, not it.
Fee Fi Faux Pas: Finn makes one when he yells at the top of his lungs at a candy party in "Ricardio the Heart Guy".
Finn: I'M NOT JEALOUS. I'M WEIRD!!! Jake:(to himself) WOOF, this is goin' bad.
Felony Misdemeanor: Daddy, why did you eat my fries? While this may have been kind of a last straw thing, it's still silly to get so upset over it. On the other hand, it may not have been a last straw thing, but more of a personal hurtful situation she remembers. Sure, the Lord of Evil could go around stealing souls and Marceline couldn't care less, but doing something like that is just personal to her. Plus, it never stated when she wrote the song. It could have been a while before he was banished to the Nightosphere. Then a later episode reveals that the fries were stolen when they were desperate scavengers in a post-apocalyptic wasteland, so maybe it was pretty much a felony after all.
Fighting Your Friend: The modus operandi of the Fight King is to lure fighters to his colosseum in pairs and eventually pit them against each other. The ghosts of previous challengers moan for the forgiveness of their friends as they fight. Finn uses the Fight King's love of this to trick him into giving him his Soul Jar, which Finn promptly smashes.
The Final Temptation: The Ice King pulls this in "What is Life?" to try to get NEPTR to become his son, Ice King takes the robot to his Imagination Zone and offers him a life of capturing (and "mating with") princesses.
"Puhoy." A Tear-Jerker as well. It's both played straight, subverted and then played straight again.
Find the Cure: "Trouble in Lumpy Space". Jake gets "the lumps" when Lumpy Space Princess accidentally bites him, so Finn and they must go to lumpy space and find a cure before sunset.
Fisher Kingdom: The City of Thieves is an interesting take on it, as unlike other Fisher Kingdoms, this one just has so many acts of thievery that you're just gonna wind up stealing something back without any actual mental or physical changes.
Foil: Ice King and Scorcher. The Scorcher's fire powers and competency contrasts with Ice King's ice powers and incompetency.
Foreshadowing: In the Ice King's genderbent fanfic "Fiona and Cake", when Cake joking puts on the Ice Queen's crown Fiona immediately smacks it away, yelling, "No! You might catch her crazy!". In "Holly Jolly Secrets Part II", it is revealed that the Ice King's crown is what caused his insanity.
In "The Eyes", where a horse spies Finn and Jake all night, at the beginning Finn remembers all the adventures they have in that week, one of them is rescuing yet another princess from the Ice King, and he screaming: I only want to be happy. The end of the episode shows The Ice King disguised as the horse spying Finn and Jake to learn how to be happy.
In "Lady & Peebles", when PB first uses her heart signature machine, there are five heart signatures on her and Lady's location. At the end of the episode, it is revealed that this is because Lady is pregnant.
In "King Worm", part of Finn's dream involves Ice King being chased by a giant monster made out of Gunther clones. Two of these Gunther mash-up monsters show up in a later episode, "Reign of Gunthers".
There also Finn's reflection in the mirror towards the end of the episode, foreshadowing the alternate universe Finn that appears at the very end of The Lich.
This one is a bit of a stretch, but in "Who Would Win" Finn mentions wanting bionic legs. While the alternate universe Finn does not have them, he does have a bionic arm.
Lemongrab is obviously very hungry in "Mystery Dungeon." For example, he asks Ice King to make himself into food, and he eats chewed-up pie out of a rat's mouth. This foreshadows "All Your Fault" where it's revealed that he starved himself because he uses all his food to make new life for his kingdom.
In "Marceline's Closet," Marceline is shown to have a diary with the name "Gunther" on it, which foreshadows her history with Ice King.
In a way, "Billy's Song" from the episode "His Hero." According to the song, Billy:
"Slayed an evil ocean" - In a later episode Finn had to face his fear of the ocean
"Cast the Lich King down" - The Lich appears in several episodes
"And the the evil Fire Count captured a damsel fair" - Possibly a hint at the events involving the Fire King and Flame Princess
"Also... he fought a bear!" - A specific bear appears in a few episodes, and that bear unintentionally helped revive the Lich for the second time
The green lady from "The Creeps" and "King Worm" foreshadows the events of "The Vault." As do all the hints about PB being The Ageless. As do all of the incarnations of Finn that lack a right arm.
It was pretty much telegraphed since season 1 that Finn would lose his right arm. He was one of only two incarnations (the other being Fionna) that hadn't lost the arm.
That is, until "Escape from the Citadel," where the arm finally came off.
Forgot About His Powers: Jake does this often when he and Finn face conflicts that would be easily resolved with Jake's growing or shrinking powers.
Jake:(farts) (fart causes avalanche and Finn and Jake are spotted) Finn:(angrily)Jake... Jake:(regretfully) I thought it would be funny...
Finn's entire adventure in "What Is Life?" is for the goal of seeing Jake get pied in the face.
Fountain of Youth: Present in the Season 2 Finale. Princess Bubblegum ends up being shattered to pieces in order to defeat the Lich who possessedher body. Not all of her pieces could be located, causing her to revert to a 13 year old girl, who looks even younger.
Four-Fingered Hands: Varies. Even the character sheets for storyboard artists explicitly state that the characters can have either 3, 4 or 5 fingers, depending on what is needed or looks good.
Billy, the legendary hero, has six fingers on both hands, at all times.
Parodied in "Five More Short Graybles", where the supposedly finger-themed stories were actually based on the five tastes. Cuber laughs at the viewer for thinking people have five fingers.
Frankenstein's Monster: Princess Monster Wife, built by the Ice King by taking various random body parts from all of the other princesses.
Free-Fall Fight: There's one at the beginning of "His Hero". Finn & Jake vs Swamp Giant.
A license plate on a sunken car briefly seen in "Storytelling" reads "AT43VR": Adventure Time forever!
The ducks feeding outside Finn and Jake's house at the beginning of "Her Parents" all have two heads.
The heart monitor at the beginning of the episode "Mortal Recoil" displays the first 15 digits of pi, "3.1415 92653 58979"
In "Hitman", if you pause at the right time you can read the note the Scorcher leaves behind when the Ice King convinces him Finn and Jake are dead.
In "Apple Thief", when Tree Trunks is held up to the mirror, the message "ï¿½- - WUZ HERE" can be seen to the side. In Morse code, "ï¿½- -" means "A. T." Guess what "A. T." means. No, not "Apple Thief"!
In "Beautopia", we see a Lub Glub shaking a skull. Look carefully at this skull; the skull is wearing a hat with bunny ears. Who else wears/wore a hat with bunny ears?
There is also a cat skeleton suspiciously similar to Cake on the pile of skulls underneath.
In "Marceline's Closet", we get a barrage of bonuses.
If one looks at the title card, you'll see that Marceline has no reflection in the mirror.
As Marceline enters her room, we can see a copy of Madonna and Child.
Marceline's journal has the word GUNTER on it.
Her trophies on a wall include a head and arm that look like that could belong to the Pale Man from Pan's Labyrinth.
In Hunson's house, there is a portrait of him golfing with Peppermint Butler.
The dictionary in "Mystery Train". It has entries for "mathematical" and "Ice King", and one of the meanings for "pancake" is "Natasha's pet cat".
Near the end of "Gotcha!", you can briefly see a page from Lumpy Space Princess's manuscript.
Simon Petrikov can be seen holding the Enchiridion in a newspaper clipping in "I Remember You".
During the opening of "The Lich" when Billy starts laughing he briefly turns into The Lich during one frame. Foreshadowing his possession by The Lich.
During "The Chamber of Frozen Blades," Finn holds up the Ice King's first edition of "Ninjas of the Ice" with the barely visible subtitle "The Art of Fridjitsu."
In the photograph of Simon and Betty seen on the tapes in "Holly Jolly Secrets Pt 2", you can see a furry figure in the background. Later, in the episode "Betty", one of the books is written by the two of them and called "My Friend the Yeti".
Also "Evicted". After a whole episode of dealing with Marceline taking over Finn and Jake's house, she gives it back... and they return to find a giant worm that blasts them with a ray of some sort and asks them to hug it. This is partially a Brick Joke, as a similar worm appeared earlier in the episode. Later on, the episode "King Worm" gives this scene something of an explanation...but see below.
Adventure Time evidently loves this trope. There was another one in "Chamber of Frozen Blades" when Gunter, one of the Ice King's penguins that he took to the hospital, lays an egg that hatches... into a floating pink kitten. Then the Ice King says, "Gunter, why didn't you tell me? Oh-ho-ho, Gunter-" as he is interrupted by Finn and Jake both kicking him.
Another example is at the end of "Her Parents" when Finn and Jake taste "Soy People," which is said to taste exactly like humans... of which Finn is the last because Rainicorns eat them. Not at all helped by Jake actually saying, "Finn, you taste delicious!" The real kicker is Finn, understandably wary at first, taking a bite of the face on his plate and then smiling agreeably.
At the end of "Mystery Train", the train falls off an open cliff and lands on a gelatin person. Jake says that this wasn't planned, and if it wasn't there they would have all died.
The most intense one yet comes in the form of "Holly Jolly Secrets" whose ending should really speak for itself. It involves a deeply disturbing and realistic portrayal of the Ice King going insane due to his crown.
The ending of "Sons of Mars" with the freed tiny manticore.
Manticore: I am the true coward, hiding from sincere expressions like a vampire in the nude who hides from the light. Thank you, brave hero. I was freed from bottle jail, but my new prison is shame. MY NEW PRISON IS SHAME! Finn: What did he say? Jake: ...He said his new prison is shame.
The ending of "King Worm". For some reason, it perfectly mirrors the ending of "Evicted!". The staff probably did that just to mess with the audience and make them think the entirety of events from that episode to this one was just a dream. AND THEY SUCCEEDED.
The end of "The Lich". Finn and Jake are dragged into an alternate universe where Jake is an ordinary dog, Finn looks like his dream mirror self from "King Worm", and Finn's mom is alive. Oh, and there's no mention of what happened to the Lich.
This episode has a followup that explains it, however.
Game Breaker: Flame Princess invokes this trope in Vault of Bones. Where Finn was going through the motion of searching for the correct way, Flame Princess could simply destroy or burn anything in her way. invoked
GASP!: Finn and Jake's reaction to the Glasses of Nerdicon in "The Real You".
Gayngst: Heavily implied with Bubble in "BMO Lost"
Also Gary... the Mermaid Queen, from "The Witch's Garden", who does not look anything like a traditional mermaid.
Gender Flip: Exaggerated with "Adventure Time with Fionna & Cake". Everyone is gender-flipped. And we mean everyone—even the background characters (e.g., Mr. Cupcake is Ms. Cupcake, the Gumdrop Lasses are Gumdrop Lads, hell, even the staffers). They even went Up to Eleven and changed the Creator Cameo of Adam Muto to Rebecca Sugar in the opening sequence.
Genre Savvy: When Finn and Jake journey to the land of the dead, Finn is cautious about drinking from the river.
Talking Skull: Yesss... drink the water. Finn: Whoa. Okay, Jake, don't drink the water! Jake: Aww c'mon, I'm so thirsty! Finn: Dude! That skull wants you to drink the water! It's bad water! Talking Skull:Don'tdrink the water. Jake:See? That means good, right?
Also in "Mystery Train", Finn concludes that Colonel Candycorn cannot be the murderer because in murder mysteries, the culprit is not usually the one suspected first.
In "Morituri Te Salutamus", Finn and Jake knew before even entering the arena that the doors would magically shut and they would have to fight a multitude of gladiators. Finn wanted to go in anyway being the Blood Knight that he is. Finn was also Genre Savvy enough to invoke Unspoken Plan Guarantee and know that the big, magical sword the Fight King had was the Fight King's Soul Jar.
In "Dad's Dungeon", Finn and Jake knew that the flower path was a trap, but they took it anyway because the other path had a really gross dude in it. It's implied that Finn also knew that the apple would kill him, but he was Driven to Suicide at the time.
Girl on Girl Is Hot: In "All Of The Little People", Finn blushes when he makes Tiny!Bubblegum and Tiny!Flame Princess Cat Fight over Tiny!him. Note that the real women would never do this and it was foreshadowing his fetish for violence.
Fionna: But there is one guy I'd do anything to date... THE ICE KING! Prince Gumball:What?! Cake: Me, too! Ice King's the REAL DEAL!
In "King Worm", when Finn and Jake wake up for the first time, Finn's hat ears are still unusually long, hinting that it's still a dream.
Go Into the Light: The concept behind "Ghost Princess" was that said character could not move on because she needed to know how she died.
Golden Mean Fallacy: Instead of choosing to wish for the Ancient Psychic Tandem War Elephant and letting Jake die, and wishing for Jake to heal while sacrificing the Ancient Psychic Tandem War Elephant, Finn uses the elephant to wish Jake back to full health and bring the Hot Dog Knights back to life.
Gone Horribly Right: In "Incendium", Jake's goal was to find Finn a new love. When he showed Flame Princess "Finn", he made her think that he didn't like her, which sent her on a rampage. She chased Jake back to the tree fort, where she met the real Finn, and slapped him for being screwy with her; Finn somehow developed a crush on her. So all in all, Jake succeeded in getting Finn a crush... on possibly the most insane, deadly, and evil princess in Ooo.
Further escalates in "Hot To The Touch", when it becomes clear that Finn is truly smitten with her. His Adorkable attempts at wooing her leads to the Goblin Kingdom nearly being burnt down to the ground.
G-Rated Sex: Maybe, in a particularly kinky scene in "Breezy" involving Finn and LSP.
Gratuitous German: Anytime Princess Bubblegum exclaims something in surprise or shock, it is likely to be in German. For instance, in "What Have You Done?", she says "Ich bin so glücklich, ich könnte, ich könnte..." meaning "I'm so happy I could, I could...".
Grows on Trees: In "The Witch's Garden" the eponymous witch has a garden full of donut bushes (and she plants a cupcake plant in the end). In "The Pods", magic bean sprouts give: adorable pigs, sparkly wands and an endless supply of icecream. The pigs are pure evil, though.
The Hedonist: Party Pat's character in "The Belly Of The Beast" suggests this. Laid-back attitude, parties and drinks honey-flavored energy drinks constantly, and doesn't seem to care or be aware that his people are in a monster's stomach and are in danger of being killed and digested.
Heel-Face Turn: Marceline's dad in "Daddy's Little Monster"... kinda. It's always "kinda" with this show.
Heroic BSOD: Jake goes pacifist in "Crystals Have Power" when he hurts Finn while roughhousing, which makes him recall a time where he did the same with his brother, and his father's response was "You're gonna hurt everybody!" Eventually he has a vision of his father, who says "I meant you'll beat up everyone evil! Let me finish next time!" Jake responds, "Oh, well, I'm over it then!"
The next episode, if you can believe it, has an even worse one. When Princess Bubblegum rejects him again, he just lies on the floor staring at framed pictures of her, and doesn't even respond when his house catches on fire.
His Name Really Is Barkeep: Doctor Princess, who breaks the show's pattern of princesses only being known by their titles (Lumpy Space Princess, Wildberry Princess, etc.) by not being an actual princess at all; "Princess" is just her surname.
Hiss Before Fleeing: Peppermint Butler senses the Lich possessing Princess Bubblegum and does this.
How Do You Like Them Apples?: The witches' apples from "Dad's Dungeon" which cause the consumer to be trapped in a giant apple to be eaten by the witches.
Hulking Out: Finn, when something happens to Jake. Then he's talked down by Jake and calms down.
Humanity Ensues: At least four different processes are shown to give human attributes to the otherwise inanimate objects, cyclops tears, power of the ice crown, lightning from "ordinary" storms and the ancient sleeping magi of life giving. Usually this will put a human-like face on the object. It is unclear how much this has to do with the general Moe Anthropomorphism of the series.
Humanlike Hand Anatomy: Played with to disturbing effect on an episode, where a deer is attacking the citizens of Candy Kingdom. During the climax the deer stands up on its hind legs and, to everyone's shock, takes off its front hooves, revealing human fingers beneath.
Human Subspecies: The more optimistic view is that humanity died out in the sense that it was mutated from its original baseline. Kind of like the way dinosaurs are extinct, but birds are still around.
The Hyena: The monster at the end of "Dad's Dungeon".
Hypocritical Humor: In "Trouble In Lumpy Space", just before Jake goes full Lumpy, he says he'll never turn his back on Finn. Then he goes full Lumpy, and the other Lumpies call him over, and he says he'll be right over, as soon as he turns his back on Finn. And he literally does.
In "Dad's Dungeon", when Jake is viewing Dad's prerecorded message, Dad says that since it's a prerecorded message, he can't hear what Jake says. Later, Dad says "Butts are for pooping!" when Jake says "But-".