The ability of animated characters to hide behind items clearly smaller than them, be it a tree, a lamp post or a drawing pin. Perhaps they disappear into hammerspace?
Sometimes two characters will lurk behind the same narrow object, clearly unable to see each other back there. Frequently the front end of one will emerge from one side, while the rear end of the second waggles out the other side. Or both front ends will emerge, spot one another, and resume the Chase Scene.
Tree Cover is a related trope, done seriously, and generally at least plausibly (courtesy of a nice thick tree).
As does security guard Bobby in Ernest Goes to Jail.
The Villain: Cactus Jack, being primarily inspired by Looney Tunes, does this at one point, with the title character and his horse hiding behind a cactus.
This is one of the "cartoon tricks" that the main character of Badly Drawn Roy can do.
Hurley tries to do it behind a table leg in G Force.
Played with in True Lies, where Tom Arnold's character takes cover from machine gun fire behind a light post that doesn't come close to covering his body. It still works, and even he is amazed he wasn't riddled with bullets.
One physical humor joke involves placing two knuckles on either side of an upright finger and asking the viewer to identify what it's depicting. The answer, of course, is "Dolly Parton hiding behind a tree." (Substitute any other well-known busty woman, as appropriate.)
The ITV Live Action children's series Woof had a variation where walking past a pole hid the main character long enough to allow him to transform into his dog form. Though this was probably due to technical restrictions on this mid-90s low-budget series.
A Calvin and Hobbes strip shows Hobbes hiding behind a standard lamp as he sneaks up on Calvin.
Lampshaded when Calvin tries hiding behind a swingset, but notices that the poles don't do a good job of hiding him.
Also spoofed in two early Peanuts strips - Charlie Brown first tries this but it doesn't work because his enormous round head protrudes on both sides of the tree trunk, so in the second example he finds a tree with a big square sign nailed to it at the right height...
Beetle has a hunch Sarge is watching him from hiding... mainly because Sarge is far too obese to fit behind the object he's standing behind, and the attempt just looks silly.
Another time, inverse to the above, Beetle annoys Sarge by letting him know he's hiding behind a pole that can hide him because he's so much thinner. (Kind of played straight, sort of accidentally, in that the pole doesn't quite look thick enough to really cover him. But it's nothing like as extreme as the usual example, and the idea is that only someone so skinny could do it.)
In one Krazy Kat Sunday strip, Krazy hides (from the stork) behind an egg roughly the size of his/her head.
Both Garfield and Odie hide behind a lamp in one Sunday strip.
Left 4 Dead: Boomers will sometimes try to hide behind thin poles. It doesn't work out as well for them as the other examples on this page.
This is a result of how the engine handles spawning more zombies; one only needs to break line of sight to spawn in so it is possible to spawn into areas that would not otherwise hide an Infected. Said poles, if the survivors are lined up, could allow one to spawn in as would the tops of cars and such.
In Spyro 2: Ripto's Rage!/Gateway to Glimmer, there's an infamous challenge where you have to follow Agent Zero to his secret hideout without him realizing you're following behind him. To do this, you have to hide behind something whenever he turns around, and that "something" is almost always a tree half the width of Spyro's body. He never sees you behind it (if you do it right, of course - otherwise, that tire screeching sound when he spots you will haunt your nightmares...).
Classical Doom games handled monsters' sight by checking whether the enemy could see the middle line of player's sprite, so this was entirely possible and very handy if, say, you needed to break Archvile's line of sight to avoid its undodgeable fire attack.
At one point, an alien ninja (and the narration points out that every culture in the universe has its own version of ninja) is sneaking down a passage, then decides to hide in a small, somewhat shady corner... which shouldn't by any rationale be able to hide him. When he gets close to it, however, the dozens of other ninja already hiding in there tell him to go away.
Parodied in Bar'd, where the entire staff of the Leafy Bar sans Shelia tries to hide behind a tiny plant from Dejero, the true boss of the joint.
In this strip for The Whiteboard, during a paintball match Bandit peeks out from behind a tree that in no way is thick enough to actually hide him, even with his normal slenderness. His peeking out almost makes it look like his head and neck are sprouting from the tree like a branch.
In The Mercury Men, Edward attempts this, hiding behind a two inch wide narrow pole.
1d4Chan, taking the Warhammer 40,000 rule that "tactical genius" Lord Ursarkar Creed can infiltrate anything (except cavalry), notes that he could hide a Baneblade behind a lightpost (a Baneblade is a tank whose stats are given in terms of being a bunker), or a Titan (vast Humongous Mecha) in a swimming pool, or an army under your desk.
Done a few times on Homestar Runner. At least twice, characters have managed to hide behind The Stick, an aptly-named local "landmark" consisting of a stick stuck in the ground. In one Strong Bad Email, "hiding", as part of a game of hide-and-seek, Strong Mad hid behind himself hiding behind The Stick. However, the fake Strong Mad was visible.
Strong Bad: Uh, Strong Mad? Buddy? That's the same spot you hide in every time we play. (Strong Mad emerges from behind "himself") Strong Mad: DAWWW! Strong Bad: Whoa! Did you make a hiding spot out of you hiding in your usual hiding spot? To hide behind? Strong Mad: I STAYED UP ALL NIGHT! Strong Bad: And it was well worth your time.
A common joke they would also have is where a character based on Frank Sinatra would be able to do this behind his microphone stand. Not as a trick, but because he's just that skinny.
In Confessions of a Nutzy Spy, Missing Lynx manages to replicate himself for a moment by looking out from both sides of a tree at once.
Played with in Ed, Edd n Eddy, "Three Squares and An Ed", which features a rare example of displaying the back of the pole. The Eds are shown to somehow be able to alter their anatomy to resemble that of spaghetti while using the banisters in Ed's house to sneak past Sarah and Jimmy (who are trying to make sure the grounded Ed stays in his room). This manages to look hilarious and oddly creepy at the same time.
Done in at least one (and probably many more) Tom and Jerry short, in which the front and rear ends of the titular characters emerged from round the side of the pole in various combinations before either of them realised what was happening.
Spoofed in, what else, The Simpsons, in which Fat Tony steps out from behind a tree sapling after hearing that the school would need construction of a wheelchair ramp.
Played straight later in "The Fool Monty" when Mr. Burns steps out from behind the microphone stand.
Spoofed in an untitled animated short, in which a character hides behind a thin flagpole. His pursuers run by, and the camera moves in behind the pole, revealing at least four other people along with the character in a lavish room with several large chairs and a fireplace. One of the others inquires who he's hiding from.
Used in the Beast Wars when the recently-baseless Predacons evade a Maximal patrol.
Subverted in a 1958 cartoon, Sick Sick Sidney. In it, Sidney The Elephant tries to hide from safari hunters by getting behind a very thin tree. However, Sidney is too fat and thus unsuccessful, saying "Oh they just don't make trees like they used to."
In The Proud Family, Oscar Proud remarks he is so thin he "disappears if he turns sideways."
ReBoot has Bob do this while inside a pirate ship's prison cell to make the pirates think he's escaped. It's implied that Mouse hacked that cell and installed hammerspace into it.
Used on Jimmy Two-Shoes, including Heloise popping out of the thin-as-a-pole Jimmy.
The Yellow Submarine does this a couple of times to hide from Ringo behind lampposts.
Pinkie Pie from My Little Pony: Friendship Is Magic is fond of this. In "Green Isn't Your Color", she repeatedly hides in places where she can't possibly fit, such as a small apple stand and a basket of sponges. She's also seen hiding under a rock in "Feeling Pinkie Keen", despite the rock still touching the ground with no gap underneath.
In the short Caballero Droopy, the villain jumps out from behind a cactus to stick Droopy up. Later, he goes behind the cactus and comes back out riding his horse.
Negaduck does this is in a Darkwing Duck episode while spying on the titular hero and Morgana.
The Tex Avery shortOne Ham's Family has the wolf hiding behind a thin tree, then coming out disguised as Santa Claus.
Parodied in Foster's Home for Imaginary Friends when Bloo is trying to sneakily follow Uncle Pockets, we see an object the perfect shape and size for him to hide behind, but he instead hides behind a lamp that doesn't cover him at all (also he's on the wrong side). When Bloo mentally revisits this scene he was properly hidden behind the lamp because he was wearing an invisibility cloak.