In a classic cartoon, any non-fatal (which is to say, just about every) head injury immediately results in a huge swelling the size of an orange or larger, usually pushing its pink, fleshy way through the victim's hair. Sometimes they come pre-bandaged. May also erupt in the exact shape of the object that caused them. Attempts to shove the lumps back into one's head will usually be answered with a lump of equal-size appearing on the other side of the head... or sometimes somewhere else entirely.
Occasionally further damage will make the lump grow into an obelisk shaped little mountain, or cause the lump to sprout another smaller lump. These can happen multiple times in a row, causing either a very tall bump, or a little cactus-like appearance.
The spherical one is more popular in Anime, whereas the mound shaped one is more popular in Western Animation, although both have appeared in either medium.
Unfortunately, a Truth in Television, as a hard hit to the head can result in a swelled bump, which, if unlucky, won't go away for a long time. Animated mediums tend to exaggerate this though.
Not to be confused with Your Head A Splode.
In the first episode of the resurrected Negima!, a mishap leaves Negi with a Cranial Eruption, among other marks.
In one episode of Bobobo-bo Bo-bobo, Bo-Bobo hits Don Patch on the head multiple times to make him get not only a lump on his head, but multiple lumps on top of that lump. Then Bo-Bobo tries to use it as a sword of some sort. When that doesn't work, he uses a bat to whack the lumps off of Don Patch's head like a bunch of baseballs. And, like everything else in that show, this carries the disclaimer .
This is common in Detective Conan, used whenever Conan gets whacked on the head for interrupting investigations.
It happened a couple times in Dragon Ball Z. One example is when Goku hit Gregory the grasshopper over the head with a mallet which he was assigned to do by King Kai. In another example, Krillin is testing Goku to see if he could deflect a rock thrown at him while in Super Sayian form; it ends up hitting him on the head and a large bump appears where it hit.
Also notable in an early episode when Goku gets hit in the head by an axe which shattered afterwards.
Occasionally happens to the title character of Naruto, especially when hit by Sakura.
One moment has Naruto get one from a log, which also gets a bump.
These happen occasionally in Galaxy Fraulein Yuna's more light-hearted episodes. One particularly amusing case in the first OAV episode had the bump "healed" by Yuri Cube snagging it off Yuna's forehead with her chopsticks — and immediately eating it, saying it looked tasty.
An amusing instance in the manga, when Happōsai's Cranial Eruption is hit by a little splash of water from the Spring of Drowned Twins. Instead of duplicating him, it duplicates his lump.
Everytime Nami gets furious in One Piece, expect these to pop up on whoever annoyed her.
And one cannot forget Luffy versus the Sphinx. When Luffy was done with it he was literally standing on the poor thing's head, atop a mountain of these.
in Niea 7, Niea gets knocked over the head by Mayuko quite a bit, leading to this. Of course, Niea deserves it every single time.
Hidamari Sketch: Yoshinoya-sensei, after saying or doing something rather un-teacher-like, is usually seen in the background of the next cut, doing the Seiza Squirm with a (often multilayered) cranial eruption while the Principal lectures her.
Miyako also receives them fairly frequently from Sae.
Yuno also gets one, though it's because she bumped her head on the moving truck.
The titular character in Crayon Shin Chan gets hit in the head a lot by his mother which always shows an off-screen cutaway sound effect.
These happen from time to time in Mai Otome, such as in one piece of official art in which Haruka, sparring with Shizuru during her time as a Coral, fails to block Shizuru's staff and gets hit in the head (resulting in the staff bending on impact). In Mai Otome Sifr, Lena gives one to Elliot while repelling her advances.
K-On!'s Ritsu is usually the one to receive them from Mio, sometimes stacked. In the anime, the stacked version (one hit for making noise while studying is being attempted, and one for poking the sole of Yui's tingling foot) is seen shrinking back on its own.
When Ritsu and Tsumugi hung out together for the first time without any of the others, Tsumugi asked Ritsu to hit her, taking it as a form of physical intimacy that Ritsu and Mio practice. Ritsu initially can't bring herself to do it, but Tsumugi later gets her wish after telling Ritsu that she was "a really good date", and that if she were a boy, the girls would be crawling all over her. At the end, there's a picture of her with it, captioned "Me and My Lump".
Strawberry Marshmallow: For a girl who does things that provoke people to knock her face down on the floor, Miu gets them surprisingly rarely. The first time in the manga was the result of Ana whacking her with a recorder for revealing her surname. (The analogous scene in the anime didn't have one.)
It happens to Tulip the giant in Jack to Mame no Ki when Jack smashes a wedding decoration over his head.
Happens sometimes in Haruhichan. Despite Nagato and Achakura giving this to each other, there are some episodes to which Achakura also gets this, such as hitting her head on the ceiling.
In one episode of Panty & Stocking with Garterbelt, Stocking walks off screen with a large metal pipe in her hand in order to confront Garterbelt about what is taking him so long to make dinner. The screen shakes and you hear a few blows being exchanged between them. Stocking walks back on screen with a knot that is equally the size of her own head.
In the Nintendo Power comic Super Mario Adventures, one episode has Mario tied up and injured, with a single, a double, and a triple Cranial Eruption growing through his hat.
Appears frequently in Italian Disney Comics (known as "bernoccolo"). Carl Barks's story "The Mad Chemist" has Donald Duck hit upon a one-in-a-million chance of one turning him into a genius. Too bad they fade away in time.
In Wreck-It Ralph, Fix-It Felix receives one on his forehead, as one of the many facial injuries he receives from Calhoun hitting him (it's the sixth or seventh time he gets hit).
In Super Mario World, one of the world victory gags has Mario blow up the castle, only to have it launch directly into a bump-shaped hill, which then rises a little bit and sports a bandage for the rest of the game, looking like a humungous Cranial Eruption.
In the Paper Mario series, you can find a rare creature called a Whacka. Hitting it on the head causes it to produce (and drop) one of these. It's actually one of the best and most profitable-to-sell items in the game. Downside is, you can only get so many of these from a single wacka before it disappears forever, and there's only one in each game...
In Rayman: Raving Rabbids, the entire object of one of the minigames is to give a Rabbid the biggest lump possible. After a set time of whacking him with a hammer, the lump just grows taller and taller, measuring out how many times you hit him. This is probably the most extreme example of this trope, with the lump easily growing many many times his height.
Sometimes used in Spongebob Squarepants, such as in one episode, where a dozen or so objects on a shelf land on Mr. Krabs' head, and then a tall lump grows from his head, located right between his eyes.
In quite a few episodes of Tom and Jerry. Spike the Bulldog is probably the most common victim of this, usually while caught in the crossfire between Tom and Jerry's antics, leading into the gag where Tom sheepishly tries to push the bump down only for it to grow back. But it becomes much more frequent in the Chuck Jones era.
An amusing Cranial Eruption happens in the Donald Duck short Fall Out, Fall In. As Private Duck builds his tent, he's shot like an arrow headfirst into a tree. The resultant bump makes a dent in his helmet. (See image above.)
Averted slightly when Pete disguises himself as the young bunny's mother. Pete is asked for lumps in her coffee, but Pete was armed with an ACME Stove Lid on her head. Three guesses who had an ACME Stove Lid Lifter on himself.
Subverted and played straight in The Wise Quacking Duck: Offering sugar for coffee, Daffy asks Mr. Meek how many lumps his wife usually gives him. Mr. Meek takes off his hat and reveals a lump on his head. Then Daffy gives him another.
In "Sahara Hare", Yosemite Riff-Raff Sam hits his camel over the head, and as the lump grows, its hump shrinks.
Frequently happens in Ren and Stimpy almost every time someone receives a blow to the head, one example in an Adult Party cartoon episode Ren hits a duck over the head with a club causing a very tall bump to form on his head and they climb it to enter the house they are going to rob.
The comic book played with this in the special "Masters of Time and Space" issue. Ren gets hit on the head twice, causing him to have two Cranial Eruptions. When another Ren, having just dropped in from a time warp, laughs at him, the first Ren bops his head, causing a bump to sprout from that Ren's head. This, in turn, creates a third bump on the first Ren's head.
In Goofy Gymnastics, Goofy gets one of these on the noggin after a set of chin-ups, when it's revealed that the horizontal bar was loose, and it promptly falls on his head. He even pushes the lump down and flexes his bicep, which makes it pop up there.
This has happened to Homer in The Simpsons on a few occasions when he gets hurt, most notably when he fell off Springfield gorge the first time he completes the fall the skateboard he was riding on hits his head producing one of these, a deleted scene reveals the second time it happens the stretcher hits his head producing a second bump.
The Flintstones. Fred gets one for storing his bowling ball at the top of the closet.