Genre-Killer: For a time, starting off with the Disney Silly Symphonies who introduced this trope, there were many Looney Tunes cartoons which consisted of inanimate objects coming to life when a store (usually a bookstore or a 1930s-style grocery store/pharmacy) closed up shop for the night ("Goofy Groceries", "Have You Got Any Castles", "A Coy Decoy", "Speaking of the Weather", etc.) The subgenre of cartoons, at least when it came to Looney Tunes, officially came to an end with 1946's "Book Revue" which, coincidentally, was also the last cartoon Bob Clampett got credit for. In a subversion, "Book Revue" is actually the best of this subgenre.
In "High Diving Hare", Sam finally has Bugs Bunny tied and standing on the edge of the platform, with Sam sawing away at the board, gloating: "Now ya smarty-pants, let's see ya get out-in this one! This time, you're a-diving!" However, as soon as Sam cuts through the board, it is the ladder and platform that falls, leaving the cut plank suspended in midair. Bugs turns to the camera and cracks: "I know this defies the law of gravity, but, you see, I never "studied" law!"
At the end of "Fastest with the Mostest", Wile E. stares at the Road Runner, still standing on a floating piece of rock, much to Wile E.'s confusion. He pulls out a sign that says, I WOULDN'T MIND, EXCEPT THAT HE DEFIES THE LAW OF GRAVITY!, but the Road Runner holds a sign that says, SURE, BUT I NEVER STUDIED LAW!, as he speeds away.
Gratuitous French: In every single Pepé Le Pew cartoon. For example, at the start of Wild Over You, an announcer calls out, "Avec, avec!" which translates to "With, with!". Probably also a case of reverse Engrish.
Grey-and-Gray Morality: The hotel owner from Porky Pig's Feat is seen as evil because he prevents Porky and Daffy from leaving the hotel without paying, and is very rude about it, but as the hotel manager, he has a right to be angry that Porky and Daffy are trying to escape without paying their bill, which is the fault of Daffy betting away the money they were supposed to pay with.
Hair-Trigger Avalanche: Demonstrated in "The Iceman Ducketh" when Daffy accidentally sets off an avalanche by shouting.
Halloween Episode: "Broom-Stick Bunny", "A-Haunting We Will Go", and "Corn on the Cop" all take place on the holiday.
Halloween Special: Bugs Bunny's Howl-oween Special is one of those cut-and-paste specials from the '70s, incorporating footage from such shorts as the first two mentioned above as well as "Hyde and Hare", "Hyde and Go Tweet", "Claws for Alarm", "Transylvania 6-5000", etc.
Hat Damage: Done to Foxy in "One More Time" and Daffy in "Ali Baba Bunny".
The Hat Makes the Man: In "Bugs' Bonnets", random hats fly by and land on Bugs' and Elmer Fudd's heads, altering their behavior to match each time.
Have a Gay Old Time: Lots of the dialogue in the cartoons were written back when their meanings were innocuous. Just remember that there was a time when "gay" meant "happy and lighthearted," a "dick" was a police officer or a police detective, a "pussy" meant a cat, and "making love" more or less meant just kissing (though the Pepé Le Pew cartoons kinda blurred the line with that one), so if you hear any of these words in the cartoon, don't stick them in the Getting Crap Past the Radar page (unless you're sure it's a bona fide Double Entendre).
Another case is Bugs' reference to Elmer as "a poor little Nimrod." It was originally a sarcastic reference to a king mentioned in The Bible as a mighty hunter. However, between the king's relative obscurity (some interpretations place Nimrod at the center of prominent events like the building of the Tower of Babel, but there is dispute on those) and the fact that Bugs is being obviously insulting, most people think that "nimrod" is generally used to mean "stupid" rather than "capable hunter," and, just like above, most people aren't even aware that calling Elmer "Nimrod" is supposed to be a joke.
Henpecked Husband: Daffy in the appropriately titled "The Henpecked Duck". Daffy again in "His Bitter Half" and Yosemite Sam in "Honey's Money".
Here We Go Again!: In "Greedy For Tweety", immediately after Sylvester, Tweety, and the bulldog are released from the hospital, they start chasing each other again. Nurse Granny notices this while looking out the window and places the patient cards back in the "in" slots in anticipation of the three being injured again.
Granny: Que sera sera.
In "D'Fightin' Ones", Sylvester and the bulldog finally are free of being linked together after a train cuts their chains. They celebrate after falling into a garbage dump... only to discover they're now connected by the legs by a pipe. They hop down the road as the cartoon ends.
Heroic Wannabe / Hero with an F in Good: Daffy Duck as Duck Twacy, Drip-Along Daffy, The Masked Avenger, Duck Drake, Stupor Duck, China Jones, Boston Quackie, Joe Monday, Doorlock Holmes, Robin Hood, Duck Dodgers, etc.
Herr Doktor: Dr. Oro Myicin, a psychiatrist from "Hare Brush", who convinces Bugs Bunny he is really Elmer J. Fudd, Millionaire (the real Fudd having run off after tricking Bugs into switching places with him) using psychotropic drugs of some kind on the rabbit.
Hoist by His Own Petard: In one Daffy Duck episode, Daffy messes around with a caveman who wants to eat him. The episode ends with Daffy tricking the caveman into poking a hole in a giant duck balloon, and the resulting explosion causing both of them to turn into angels.
Daffy: You know, maybe that wasn't such a hot idea after all.
Hollywood Magnetism: The short "Bugsy and Mugsy", culminates with Bugs putting roller skates on a tied-up Mugsy, then using a magnet under the floor to move Mugsy around...and slam him repeatedly into Rocky. In reality, the magnetic field wouldn't be strong enough to pass through that much wood.
Also, Daffy frequently played the role of a pushy door-to-door salesman strong-arming a reluctant character into buying unwanted goods, as in such cartoons as "The Stupor Salesman" and "Design For Leaving".
How We Got Here: "The Old Grey Hare" features a sequence of Elmer and Bugs as babies when they first met.
Humanlike Foot Anatomy: Spike and Hector, the two bulldogs, Sylvester the cat, and most other cats and dogs in are shown plantigrade.
Porky Pig averts this trope in The Looney Tunes Show by having the unguligrade stance that real pigs have, but he usually appears more digitigrade or plantigrade. Also, Porky Pig normally has feet and hooves shaped like slippers.
The bird characters Daffy Duck, Tweety, Foghorn Leghorn, Henery Hawk, and Yoyo Dodo have a plantigrade stance, as do most other bird characters. Averted with Roadrunner since he keeps the digitigrade stance that real birds have.
Humiliation Conga: There're a lot of examples, but the best one is an early Chuck Jones cartoon called "Good Night Elmer", one of the few cartoons to have Elmer as the star, rather than the antagonist. After doing everything he can to get some sleep — including nearly destroying his room — what should appear outside his window but the sun?
The Hunter Becomes the Hunted: Three Pepé Le Pew cartoons ("For Scent-imental Reasons," "Little Beau Pepé ," and "Really Scent") end this way, as does "Rabbit Fire" (the first installment of the "Rabbit Season/Duck Season" trilogy) with Bugs and Daffy hunting Elmer after it's revealed that it's neither Rabbit Season nor Duck Season — it's Elmer Season.
Hurricane of Puns: The Merrie Melodies classic "Have You Got Any Castles?" I mean, the climax of the film's final chase scene ends with Rip Van Winkle opening up a book literally labelled Hurricane which blows everybody away...and then after everyones gone, down falls the book Gone with the Wind.
A key criticism towards Lola Bunny in her debut in Space Jam, who despite being boosted a new leading character to the franchise, played very little part in the cartoony antics of the original cast (to the point even some of the live action characters fall victim to squash and stretch slapstick more than she does). The one instance she is put at harm by one of the Monstars, it is Played for Drama and averted by Bugs performing a Heroic Sacrifice. The character was revised for The Looney Tunes Show, with the character having a more abrasive personality, albeit still mostly in a dialogue centric sense.
Wile E. Coyote and the Road Runner. The Road Runner is one of the few regulars to never be the butt of a gag. While most Looney Tunes protagonists are more frequently dishing out slapstick abuse than taking it, they at least have some exceptional cases. The Road Runner's most distinguishing wacky characteristic was holding up a sign reading his opinions.
Impossible Insurance: In "Fool Coverage", Daffy is an insurance salesman trying to sell Porky some life insurance. He promises the policy will pay Porky one million dollars for a black eye... provided it was the result of an elephant stampede happening in his house between 3:55 and 4:00 PM on July 4 during a hailstorm. At the end of the cartoon, that is exactly what happens! To try to save face, Daffy adds "and a baby zebra" to the clause. Cue baby zebra.
A variation of this occurs in "Boobs In The Woods." After asking Porky if he has a fishing license and a hunting license, Daffy asks if he has "a license to sell hair tonic...to bald eagles...in Omaha, Nebraska." Porky does, oddly enough.
I Need to Go Iron My Dog: In "You Were Never Duckier", when Daffy Duck(impersonating a rooster to cash in on a poultry contest) encounters Henery Hawk's very large father, wrongly thinking that he was the judge of the poultry contest:
Daffy: Whoops, gotta go, my judge is burning, fudge, I mean, my fudge is burning, judge, my mother wants me, er, I gotta crochet a cake, um, g'bye!
Indestructibility Montage: In Much Ado About Nutting, a squirrel spends the entire cartoon trying to break open a coconut. Nothing it tries works, culminating in him dropping it from a tall skyscraper and only succeeding in pounding in the pavement below. Once it finally does crack (with only minimum effort), the shell opens to reveal another shell.
Ineffectual Sympathetic Villain: Arguably a Trope Codifier, as almost every villain in the series was a moronic Butt-Monkey as likely to fall by their own idiocy as by the actions of the protagonists themselves. Even the rare subversions of this trope (eg. Nasty Canasta, Rocky and Muggsy) ultimately suffered Villain Decay and fell victim to it.
The Coyote was, in fact, so sympathetically ineffectual that in many viewers' minds the Road Runner became the real villain of the pieces. Hilariously referenced by "Weird Al" Yankovic in UHF:
"Okay. Right now I'd like to show you one of my favorite cartoons. It's a sad, depressing story about a pathetic coyote who spends every waking moment of his life in the futile pursuit of a sadistic roadrunner who mocks him and laughs at him as he's repeatedly crushed and maimed! Hope you'll enjoy it!"
Inflating Body Gag: In the early short "Hold Anything", a goat bites into a radiator, causing the hot air to inflate it and send it flying like a balloon. Bosko then uses it as an improvised set of bagpipes.
Instant Gravestone: There's a gag in the short "Baseball Bugs" where a player tries to catch a fly ball ("I got it! I got it!"), gets plowed into the ground by it, and a gravestone pops up where he stops (reading: "He got it").
Instrumental Theme Tune: Sort of. The iconic theme songs, "Merrily We Roll Along" (for Merrie Melodies) and "The Merry-Go-Round Broke Down" (for Looney Tunes) do indeed have lyrics, but they're never used when introducing the shorts. All we hear are the instrumental versions of them.
"The Merry-Go-Round Broke Down" used lyrical variants in Daffy Duck And Egghead and Boobs In The Woods while "Merrily We Roll Along" was performed by an animated Eddie Cantor in Billboard Frolics and Toy Town Hall. Before becoming its theme, "The Merry-Go-Round Broke Down" was used as background music in a segment of "Porky's Garden" (Avery, 1937).
The 1939 short "Porky's Movie Mystery" features The Invisible Man eating an apple. Thanks to Cartoon Physics, the chewed-up apple reassembles itself in his stomach, where it remains for the rest of the scene.
In the 1952 Bugs Bunny short "Water, Water Every Hare", Bugs turns himself invisible with a bottle of "Vanishing Fluid". While invisible, he chews and swallows a carrot, and the audience can see it being ground to orange meal and falling down his throat.
Involuntary Dance: In The Wearing of the Grin, Porky is forced to put on some green shoes which make him dance through a nightmare landscape.
Iris Out: Done at the end of most shorts (the practice lessened around the early '60s, when the series opted for a simpler "fade to black"). In many Bob Clampett shorts, the "iris out" was often accompanied with a cartoony "Beeeuuuyyywwooooooo!" sound effect. A couple subversions:
A Fractured Leghorn: The short does an "iris out" during Foghorn's rant. He grabs the iris so he can finish.
Foghorn: Wouldn't tell 'em I was hungry!
Duck Amuck: Daffy, exasperated, says "Let's get this picture started!", to which the short does an "iris out" and "The End" appears. Daffy yells out two Big Nos and pushes the ending card off screen, and the cartoon continues from there.
''Hare Ribbin'" has the dog, after having committed suicide, suddenly rising, stopping the iris out to say "This shouldn't even happen to a dog!", and then the iris out closes in on his nose.
Porky The Rainmaker (1936) has the iris closing and a farm duck is inside the black area. He bangs on the darkness, then Porky's arm reaches in and pulls the duck back to the outside.
Porkys Garden (1937): Two irises re-open as Porky takes the prize money from the Italian chicken farmer.
Ballot Box Bunny (1951) has the iris close in as Bugs takes his turn at Russian Roulette. It opens back up on him to show he ducked out of the way of his shot, then another iris opens to show the shot hit Yosemite Sam.
It Wasn't Easy: In "The Super Snooper", when Daffy Duck (as detective Duck Drake) is told by a Beverage Hills mansion butler to "walk this way" and mimics the butler's peculiar manner of walking, he turns to us and says, "T'ain't easy!"