Bleach: The Hougyoku is a simple, tarnished orb when it's first introduced. Aizen even remarks that it's strange that such a simple object could be so powerful.
InuYasha: If it's a simple-looking orb you're looking for, look no further than the Shikon no Tama (Jewel of the Four Souls), which is the size of a large marble. Sealed within it are the souls of a demonic horde and priestess trapped in an eternal battle. Even a tiny shard of it can make give a demon or human a powerful upgrade, and the first time we see it it's represented as a tacky key-chain. It's eventually revealed to be the true villain of the series, manipulating even Naraku (Naraku already knew that but the others only realised at the end of the manga).
In the Orichalcos arc of Yu-Gi-Oh!, Joey inherits a card that can fuse with a monster to create never-before-seen equipment cards. During its first use, it creates a winged gold croquet hammer with chibi wings. Turns out it has the power to send all of his opponent's monsters forward in time by a random amount. His opponent is left defenseless, which earns a raw beatdown, and the monsters only come back one by one, allowing the members of the former army to be picked off.
In Cardcaptor Sakura, one of Eriol's plots involves turning Sakura's homemade teddy bear into a booby trap.
The Clow cards, themselves, are Happy Fun Balls. They look harmless, but can cause destruction if they're not controlled by a magician.
The Joker resorts to these all the time, when he's not directly using a clown- or comedy motif. Even then, it's pretty common he'll resort to something that actually is harmless as a fakeout, knowing people will assume the worst anyway.
In the comic version of The Tick, one villain incapacitates The Tick with a device called "the Happy Apple".
The "Lok-nar", the evil green sphere in Heavy Metal, had a tendency to melt the flesh from the bones of people who happened to touch it, when it wasn't busy animating corpses. Although it doesglow. That should have been a giveaway.
In the DCAU cartoon, he once used a literal happy fun ball, which totalled an armored car.
The Ultimate Nullifier of the Marvel Comics universe is about the size and shape of a cell phone, curved on three sides and flat on the fourth, with a couple widdly little protrusions. It could destroy the universe if used right, and will likely destroy the user whether used wrong or right. Contrast, say, the Infinity Gauntlet, which looks exactly like the kind of glove you would want for punching gods in the face.
Richard Corben wrote a short comic called "Top to Bottom" where a children's toy that looks vaguely like a high-tech 3D labyrinth game proves to be a potential world-wrecking device when used unwisely. Borderline terrifying.
In Fallout Equestria: Duck and Cover, protagonist Atom Smasher carries a nerf gun, which is about as dangerous as you'd expect. Until some traders in need of extra security provide her with a clip of reusable darts enchanted to explode on contact. Given that Atom Smasher has no form of empathy whatsoever (she's intended to reflect the typical Fallout protagonist as played by an immature gamer more interested in killing baddies and finding cool stuff than acting like a decent moral being), this means trouble for everypony around her.
Childs Play is built around this premise, with a serial killer's soul trapped inside a "My Buddy"-esque doll.
Transformers movie: An innocuous-looking radio proved to be the Decepticon Frenzy. And the movieverse version of Frenzy is even more terrifying. When he's eventually knocked in half, what's left of him is enough to, with a bit of shifting as parts, function as a killer robot and hide as an equally innocent-looking cell phone. More than Meets the Eye, indeed.
The Continuum Transfunctioner from Dude Wheres My Car, a device with the power to destroy the entire universe, spends most of the film disguised as an ordinary Rubik's Cube.
The puzzle box from Hellraiser series. Solve it, and eternal pain/pleasure can be yours.
The climactic battle scene in Toys is built around this premise.
In Kentucky Fried Movie, during the kung-fu spoof, the main character sends a seemingly-innocent toy robot into the room. Most of the bad-guy scientists jump out the windows, screaming. "Toy Robot? Ah! A toy robot!" (This is a reference to Enter the Dragon, where Bruce Lee uses a snake to much the same purpose.) When one of the scientists realizes it's just a toy robot, a machine gun pops out of its torso and riddles him with bullets.
The scientist makes the mistake of ridiculing the toy robot. As it shoots him, it says, "Eat lead, sucker" in a growly voice.
In Men In Black, when J is getting a tour of the office, he sees a small ball floating above a machine. When he touches the ball, it goes careening at high speed through the whole building, smashing windows and knocking people over before K catches it and puts it back, explaining that it had caused the 1977 blackout in New York.
The ball was meant for fun, apparently a alien ambassador released it as part of a practical joke likely not realizing it would cause a massive blackout. He thought it was funny as hell.
To a lesser extent, the Noisy Cricket is one as well. It looks like a child's toy Ray Gun and is smaller than J's hand ("I feel like I'ma break dis damn thang!"), but it is apparently the most powerful sidearm in the film.
According to the powers that be, the Noisy Cricket was originally designed as a concealable weapon, but proved to be easily modified to the magnitude it exhibits in the movie. This was done as a form of hazing for new recruits.
The Galaxy itself.
In Castle in the Sky, Sheeta's Laputan grandmother taught her the "Spell of Destruction" as a Nursery Rhyme on condition she never use it. Which she inevitably does when duly provoked by the Big Bad.
In Dark Star, there is the Happy Fun Beach Ball. An (almost adorable) precursor for the alien in Alien and technically a Killer Rabbit, but still ...
In Day Watch, Yegor, a powerful dark magician uses a foil ball as his weapon of choice. Seems harmless, right? Until he uses it to slaughter almost everyone at his birthday party, blind his light equivalent and pretty much destroys all of Moscow.
In the Korean horror film The Red Shoes 2005, the titular shoes look like just an ordinary pair of women's high heeled shoes, but the shoes are haunted by a murderous ghost. Whoever touches or wears the shoes is driven to insanity and murder.
In The Rock, the opening scene is the hero and a colleague attempting to defuse a child's doll stuffed with C4 and spewing poison gas.
In the Tank Girl movie, there is a murder-ball - a harmless little mental ball but if you hold it wrong, spikes will come out and shred your hand, pretty much putting you out of the rest of the fight.
Interstellar Pig has "The Piggy", a small grinning pig statue. Cute and harmless-looking, but it's likely to destroy every planet in the universe except the one it's on when the time is right. Then the Piggy admits it made up that legend and says it only destroys the planet it's on... when it hiccups. Poor piggy, it just wanted friends... At the end of the story, the reader learns that the Piggy is nothing but a recording device that made up the stories to be passed around and record everything. It can't destroy anything — although it can teleport anything touching it away if it becomes bored.
In Stephen King's Desperation, a hand-held statuette of a poorly-carved coyote, touched by the Big Bad, has the adverse effect of causing those in contact with it to give into their savage, normally restrained instincts.
While turning the savage Up to Eleven, from the account of several characters.
The first Gaunt's Ghosts novel, First And Only, plays with this. In one scene, an enraged Ghost shoots an innocuous but blasphemous Chaos idol to pieces; one of the pieces lodges into another Ghost's skin. A while later, that same Ghost goes insane, starts shooting his comrades, and then mutates into a horrific spawn of Chaos, created by the corruption embedded into the idol's fragments.
In the Liaden Universe books, a Bowli ball is a ball with a gyroscopic randomizer that can send it flying off in random directions. It is used as a combination game and exercise for pilots, and has been known to lead to physical injuries.
In Louis Sachar's Sideways Stories From Wayside School, Todd brings a sickeningly adorable toy puppy to school for show-n'-tell. The puppy can be manipulated into a terrifying, twisted creature that resembles a werewolf. Joy steals the puppy, not knowing this, and is bitten by it. The puppy toy doesn't let go.
In Keys to the Kingdom, the most powerful Key is... well, a simple skeleton key, albeit gold-colored & probably glowing impressively.
In Soon I Will Be Invincible, the villain Dollface is said to have specialized in innocuous-looking dolls full of powerful miniaturized super technology. Dollface herself is only mentioned, but her work - a gravity manipulating doll to be more precise - is used in DoctorImpossible's doomsday device to move the moon at will, controlling the Earth's orbit and thus the temperature of the Earth.
In a later episode, they had an Unfrozen Caveman Lawyer sketch which included Happy Fun Ball among its humorously fictitious sponsors: "Still legal in 12 states!"
This trope may stick to certain kinds of skin.
If the outer shell breaks open, avert your eyes from the core and seek shelter immediately.
Power Rangers Zeo had a cute little dog statue that turned out to be the Midas Hound, an artifact that turned things (and people) to gold at random. It was eventually made larger, gilded the good guys' Humongous Mecha, and then became a Transforming Mecha of its own!
Warehouse 13 gives us the Baylor Dodgeball, a fairly innocuous-looking dodgeball previously used in military training exercises. It bounces under its own power and plays dodgeball with whoever happens to be nearby. Sounds harmless, right? Wrong. First, it "throws" itself only when its victim isn't looking, so there's no chance the person will catch it without a partner. Second, it creates a copy of itself when it connects, and each of those copies acts exactly like the original. That means you'll have dozens of them in no time flat. To stress why this is bad, it bludgeoned five soldiers to death before it was caught. However, all you have to do is catch one of them and it returns to being a single, inert dodgeball.
Come to think of it, almost everything in the Warehouse is a happy fun ball/artifact of doom. Some...not so much, like the Mayan idol from the first episode.
The Weeping Angels, innocent looking statues of angels with their hands held up to their face as if they're weeping. But when nobody's watching, they spring to life and start killing indiscriminately. That's why you must never Blink.
Oh, and you can't ever film, draw or otherwise depict them or else the depiction will become one. Which leads to Fridge Horror when you realise that is exactly what the show itself does. And if you look into one's eyes for too long, your visual memory of it will become one and murder you from inside your brain.
Reaper had a foam dart gun. The key difference between this gun and any other Nerf gun is that this thing could send anything to Hell. Extra emphasis on the word anything.
In Space Cases, a girl brought on board a cute little teddy bear and gave it to a friend. It turns out that the bear is a trap that has a deadly virus on it.
Firefly has Zoe explain why she always cuts her apples up with a knife before eating by telling a story from the war. They were in trench warfare, and were running low on food. Their lack of food came up while talking with the enemies in the other trenches, and shortly afterwords apples started falling into their trenches. Apples with grenades in them.
Ask any veteran Dungeons & Dragons player about the Deck of Many Things. Doom of the multiverse, in an innocuous deck of tarot cards.
The Necklace of Strangulation is a particularly infamous example, to the point where Genre Savvy players won't loot a necklace from a corpse until it's been identified. Indeed, players accustomed to the older editions of the game won't touch any item that radiates magic until they've had it examined by a sage... and even then, using an unfamiliar item for the first time is a moment of some trepidation.
The same item shows up in NetHack, but it doesn't stand out quite as much; this is because you get used to being paranoid about everythingin NetHack.
There is even a golem type in the Second Edition Monstrous Manual called a "doll golem." It is an enchanted child's toy that can be designed to do various different things. Common roles are either to murder the child in question... or far more amusingly, murder anyone attempting to harm said child.
Ravenloft has an entire town of this trope called Odiare, where Evil Pinocchio regularly murders all the adults too old to play with him.
There's also the Bag of Devouring - seems to be an ordinary sack, or possibly a Bag of Holding. It's actually the mouth of an extradimensional Eldritch Abomination. Store your magic items in it? You'll be very disappointed when you try to retrieve them. Stick your hand in it? You'd better have a damn good Grapple modifier.
Indeed, D&D's original name should have been 'Happy Fun Ball: The RPG.' Killer statues, killer walls, killer floors, killer bridges, killer ceilings, killer stalactites, killer cloaks, killer bedsheets, and killer water can be found with a glance through the bestiaries. One cursed magical item worthy of mention is the Scarab of Death, which has the form of a scarab (Egyptian-style brooch) until sustained proximity to a body warms it up, at which point it becomes the other kind of scarab (a flesh-eating beetle) and eats its way to the host's heart.
Mage: The Ascension's final supplement, Judgment, features a scenario concerning the ultimate fate of the universe. To avert the destruction of the natural flow of reality, the players must discover what sort of catastrophe damaged the world to this degree and the weapon that destroyed the universe once before named the First Tool... which is an obsidian hand axe of the simplest form.note However, the very final events of the story includes the First Tool becoming reunited with its spiritual counterpart, which is the concept of violence. Once restored, it can destroy the universe once again. Perhaps a subversion?
The card game Munchkin has a card called "Duck of Doom" which immediately drops the affected player by two levels (the point of said game is to reach level 10). The card's caption reads, "You should know better than to pick up a duck in a dungeon".
West End Games' TORG had "Conjure The Bouncing Hordes Of Doom", found in the supplement "Pixaud's Practical Grimoire". The material component for the spell was a rubber ball with arcane symbols carved into the surface. When the caster tossed the ball while saying "I invoke you", the ball would split into six armed and armored Munchkins with Speedball's bouncing powers and resistance to kinetic damage, combined with Wolverine's skills with blades AND a gremlin's sheer nastiness.
In the New World of Darkness, you have multiple different kinds of magical items that can appear completely innocuous while actually being a source of terrible power. The most likely ones to take the form of a Happy Fun Ball are changeling tokens, for two reasons: first, tokens are concealed by a Mask so as to appear completely innocuous, and second, they're the easiest for a mortal to activate (accidentally or otherwise). Of course, doing so without paying Glamour means that you're going to invoke its Catch...
As an example, the Blood Pennon just looks like a strip of sackcloth tied to a stick. Wave it around, however, and you can give yourself and all your allies an extra boost of combat power. Activate it without paying the Glamour cost, however, and it'll leave you drained afterwards... and it'll alert the Gentry as to your presence.
There's a literal one in Final Fantasy VI. The superball item deals a fair bit of damage when used.
Dwarf Fortress is FILLED with these. The throwing mechanics are so broken, throwing laundry around will kill all involved, while passing Necromancers will often create an army of walking Fluffy Wambler corpses just to piss you off. And then there are the Giant Sponges - with no nervous system, the only thing they can feel is ANGER.
The strategy/RPG Vandal Hearts II has, in addition to swords, spears, et cetera, an entire class of "oddball" weapons like baseball bats, scythes, and so forth. Most of them are genuinely threatening, if weird...until you get the "Evil Doll," which is a simple children's doll. Wearing a dress. Not only is it one of the most powerful weapons in the game (characters attack by holding the doll in front of them by the head; it shakes once and enemies die, complete with huge gouts of blood), but it teaches its holder a spell that sets your entire party on fire, bringing horrible death to allies and enemies alike.
In Planescape: Torment, one of the player's past incarnations kept a journal disguised as a pretty, dodecahedron-shaped puzzle box. The puzzle box was loaded with blades that slit a user's wrist, canisters that unleashed the fantasy equivalent of nerve gas, and other delightful traps for the would-be diary snooper. Imagine a Rubik's Dodecahedron that punished you lethally when you made a move that did not get you closer to solving it and had no visual clues to tell you when you were going the right way. Since you play an immortal with massive regenerative abilities, you are the only one who can open it.
Parodied in Portal: The Enrichment Center reminds you that the Weighted Companion Cube will never threaten to stab you and, in fact, cannot speak.
And then played with in a mod where it will, in fact, stab you.
Given the chance, it seems that everything produced by Aperture Science will try to kill you. Especially if it can be found in The Enrichment Center.
Cave Johnson: Oh, in case you got covered in that repulsion gel, here's some advice that the lab boys gave me: Do not get covered in the repulsion gel. We haven't entirely nailed down what element it is, yet, but I'll tell you this, it's a lively one, and it does not like the human skeleton.
The Familiar Spell in Ironsword summons a tiny little happy face that flies around and turns enemies into coins. One can only hope that this process is lethal, lest it become a case of And I Must Scream for the unfortunate wildlife.
The Flak Cannon from the Unreal series fires giant explosive shells with a friendly looking yellow smiley face on the front.
Unreal Tournament 2004 displays the message "Player A played with Player B's happy fun ball..." whenever someone killed another player with the Grenade Launcher.
"Happy Fun Ball" is also the Fan Nickname of the bouncing, giggling, spherical, smiley-face adorned Proximity Mines in the Chaos UT mod, included with the Game of the Year Edition of the original UT.
The Superheros Quake mod had this as a selectable power. It was activated on the player's death, which would be followed by a handful of bouncing yellow happy-face spheres bursting from the body, bouncing and swooping with diabolical laughter, and exploding lethally on all nearby opponents (and sometimes allies.)
One of Total Overdose's special attacks is a pinata. When thrown all enemies are lured running to it with giddy excitement. Its confetti-showering explosion is instantly deadly.
In Persona 3,, there is a weapon called the "Toy Bow", a plastic bow that fires suction-cup toy arrows. During the part of the game where you get it, it's very powerful and, unlike nearly every other bow, is very accurate. However, its usefulness swiftly wanes as the game continues.
In Rakugaki Showtime you have the Smiley Ball — a doodly smiley face you throw at people. Throw it at enough people and it gets angrier and angrier looking — until it turns rainbow colored with a look of awe on its face, and becomes the most powerful item in the game.
I Wanna Be the Guy has Delicious Fruit, whose rationale for crushing you (sometimes even flying upwards) is that they're "more like giant cherries" than apples.
Liero, a real-time remake of Worms 1 in which you have lives as opposed to multiple worms, goes Serial Escalation as it allows fan-made weapons to be imported. For obvious reasons the Warhammer40k and Dune mods trump the above, but one particular 'strange weapon collection' has something called the Crazy Cannon which is a bazooka that fires a small slow-moving orb. Just as you finish thinking "That was lame" it flips out and begins bouncing around the screen like Ball Kirby, WHILE rapidly spamming rockets at max charge speed for a FULL MINUTE. 90% of the time, using this clears out all land on the stage and makes everyone die repeatedly. Also will absolutely CRUSH the processor of any computer made before 2001. Thankfully recharge time is so long that it will almost never recharge during a match, so it appears to be meant as a screw you option. If your ninja rope can reach the ceiling however, it's actually possible to survive it with some bullet hell dodging skills. Oh, what I said before? Roll that up to 2003 if you pick one for all 5 weapon slots. Bwuahahahahaha!
Fallout 3 has "rigged baby carriages," which appear to be normal baby carriages from a distance. However, get too close and a distorted lullaby plays, along with the rather creepy sound of a baby crying. Then the carriage explodes spectacularly.
And the Euclid Finder in Fallout: New Vegas, seemingly a harmless toy gun... which is in fact a target finder for a Kill Sat. If you decide to arm said satellite before retrieving the Finder from its current owner - a small child using it, predictably, as a toy - one of your companions may remark how incredibly lucky you were that the safety was still on.
Subverted in World of Warcraft with the Happy Fun Rock. You toss it to other players...and it does nothing. Really. No, really.
NetHack has musical instruments that cause large earthquakes and shoot fire and ice beams (even better than wands that do the same things, though much more rare). Anything can be equipped as a weapon but it won't hurt your opponent unless it performs damage calculations (with the exception of the cream pie: it goes "splat" if you try to hit something with it, blinding things with eyes). There's also the loadstone; small and innocuous, one of four types of grey stone items in the game, back-breakingly heavy and usually cursed if you accidentally pick it up.
Tinned food is usually a good choice of sustenance, but the contents can be almost any creature you can encounter in the game, and eating some monsters isn't always advisable. Szechuan-style cockatrice is something you only try once.
Guilty Gear features the infamous Bridget. Less famous is the fact that Bridget's weapons included a yo-yo and a teddy bear, both capable of sprouting sawblades or shooting out jets of flames.
In the backstory of The 7th Guest, villain Stauf is a toymaker who specializes in Happy Fun Balls. His beautiful toys are snapped up by local children, all of whom become mysteriously and fatally ill...so he can fulfill his half of a Deal with the Devil and steal their souls.
In The Last Express, the Firebird seems to just be a golden jewelled egg with the curious ability to transform into a falcon that can sing when you play a whistle. What you don't know is that, if you do this at night, it becomes a deadly weapon...
In Metroid: Other M the Little Birdie is a cute little white bird-looking creature, first seen unsuccessfully and humorously attempting to get on top of a large fruit, but turns out to be the juvenile form of a clone of Ridley, the Space Pirate general who decimated her home planet of K-2L, and had been kept as a pet by oblivious researchers until it dismembered one and escaped. Wow. Although the more perceptive players (and Samus herself) will suspect that it's not as it seems, especially with regard to how it appears to apparate behind Samus before she goes through the door into the Navigation Room, staring at her with its beady little eyes.
In the Dadgame, defeating MechaDeath earns you... a cat launcher. Which deals damage. And is the longest range weapon of the game, especially if you charge it up - the cat gets sent flying across the screen.
Ogre Battle has Doll Mages / Masters who, while capable of throwing the dangerous Acid spell from the rear of the formation, use a marionette to attack from the front row.
In Arcanum the animal summoned with the Call Beast spell changes depending on your Magic Aptitude. If you have 100 then you summon a Vorpal Bunny. It looks like a regular white bunny ... except it's a level 40 creature in a game where the level cap is 50 and it has master level melee skill. When it kills someone they are always dismembered in the process.
It's Walky! has the Power Booster Rod, which looks like a large tree branch, but could potentially destroy the entire universe.
Largo's "Cool Thing" from MegaTokyo looks like a toy ball with lights and buttons.
Note that the Cool Thing (TM) has not been established as any sort of universe-destroying artefact of doom, though it does emit alarmingly loud noises when provoked (what Largo does to the Thing can in no way be called "use").
Castle Heterodyne houses the Mobile Fun Units, bison-sized, murderous cat-like clockworks. When the castle is fully functional, it has complete control of them. When the Castle isn't fully functional, then you'd best hope the Mobile Fun Units are confined to one room and don't know you're around.
Hannelore in Questionable Content has a roomba. At first the most heinous thing it does is threaten the standing of Winslow. Then Marten comforts Hanners. The thing somehow pulls a knife, believing Marten was making a move on "its woman".
Any attempt to destroy the shredder by aerial bombardment would be futile.
In Rusty and Co. Presti combines several apparently harmless items (including a funnel and a Bag of Tricks) together to form a deadly "Badger Launcher".
Tom Foolery, from the Global Guardians PBEM Universe is an Expy of both the Joker and the Toyman. While he's a capable hand-to-hand fighter and a nimble acrobat, his greatest weapons include things like yo-yos with razor sharp jacks, robotic, machine-gun toting teddy bears, model airplanes carrying micro-missiles, and a bouncy explosive ball that he even calls Happy Fun Ball.
And if it isn't a Happy Fun Ball, SCP-914 can make it into one. One notable example being an enhanced super ball the killed several people, destroyed entire wings of the site, and was last spotted orbiting Mars.
At the Superhero School Whateley Academy in the Whateley Universe, there's a deviser codenamed Knick-Knack who makes these all the time. We've seen a fake 'Harry Potter' wand which has been turned into a real laser weapon, suspicious snow globes, lava lamps that are now attack devices a la 'Rover' of The Prisoner, and a conch shell with something inside it that you don't want to know about.
In the "Happy Tree Friends" episode "A Vicious Cycle" Flippy is killed by a bolt of lightning and his soul inhabits Cub's tricycle which he uses to kill those he was trying to kill before his unexpected death.
In an episode of the Earthworm Jim cartoon, a page containing a spell that could destroy the universe was, due to a grievous printer's error, placed in a children's pop-up book that happened to be owned by the protagonist.
In another, the most powerful force of destruction in the universe was locked away in a snow globe.
In Ben 10, the Omnitrix appears to most to be a regular wristwatch. It's a Clingy MacGuffin that calls up obscenely powerful monster forms for the wearer.
In the cartoon version of The Tick the Tick is incapacitated by "The World's Comfiest Chair". (No one expects a reference to Monty Python's Flying Circus in a children's animated series...)
Subverted when the Tick encounters the Infinity Ball (which looks just like an 8-ball), supposedly an "ultimate weapon". It bumped into the Tick and fell.
"Where I come from, we hit these into pockets with sticks." "GASP!" "And we have higher numbers, too!"
DuckTales had an item that was similar to the Power Rangers example above, but infinitely worse: a small goose idol that turned things into gold... at first. Then it started a chain reaction that was turning the entire world into gold. The golden goose was held in a magic fountain that neutralized its gold-turning powers. If the goose was taken and kept away from the fountain's water for too long, it will awaken and turn things and people to gold on its own accord. And if allowed to remain awakened for too long, the goose's gold-turning powers escape from its body, leaving it as a normal goose, while the powers proceed to turn the world to gold, referred to as "The Golden Death". The only way to stop this scenario, obviously, is to return the goose to the fountain.
In one episode of The Simpsons, Prof. Frink discusses the possibility of turning a souvenir he found at the airport (a motorized ball with a toy weasel attached) into a weapon. At the end of the episode, the toy rolls onscreen; Frink panics but Homer, convinced it's just a "cute little weasel", goes to pick it up. Cut to a "The End" title card as we hear zapping sounds and Homer screaming.
In a slightly more literal example, a Treehouse of Horror segment featured dolphins violently rising up against humans. One of the weapons that they used was a red utility ball. They were surprisingly effective, as Kent Brockman will attest.
The Futurama episode "Mother's Day" had a younger Professor Farnsworth invent the adorable children's toy "Q.T. McWhiskers" for Mom's Friendly Robot Company. Mom subsequently turned the toy into a laser-firing death machine for the intergalactic arms market.
Batman: The Animated Series villain Baby Doll has a variety of deadly weapons disguised as toys, including a doll with a gun inside it and explosive bouncy balls.
Joker occasionally uses these as well. His trademark lapel flower usually contains acid, and he's been known to use marble grenades.
Also, his equally signature toy gun with a "BANG!" sign, which then fires anyway. This is eventually used to kill him in a flashback in Batman Beyond.
The Venture Bros. has ...The Nozzle, an innocuous yet fear-inspiring doodad that not even its owners understand the purpose of. Its engagement is accompanied by an unsettling mechanical voice warning the patient; "Do not move while ...The Nozzle is engaging. Moving will interrupt calibration of ...The Nozzle. Please do not look away while ...The Nozzle is calibrating."
"...The Nozzle is calibrating..."
Quackerjack in Darkwing Duck tends to use these. Hell, Mr. Banana Brain is the only mundane toy of the lot, and he's still got some pretty distressing ideas.
Which become even more distressing when he is possessed by Paddywhack.
Of course, don't forget that one version of Mr. Banana Brain was an inflatable failsafe device meant to destroy Quackerjack's toy kingdom, and several other versions were *cough* grenades.
Perhaps in a direct reference to the SNL sketch, Superman: The Animated Series villain The Toyman uses a happy fun ball on some Mooks that eventually moved so fast, and hits so hard that it dented the armored car it bounced against. And being Toyman, most of his gimmicks are Happy Fun Balls.
"Uh, maybe you should read the warning? Dopey Dough is a lethal biogenic organism. Contact with the skin can prove fatal. It won't stop growing until it asphyxiates its host. NOT for children under three."
In the Grand Finale of Justice League Unlimited, Toyman is surrounded by Parademons. He promptly shoots them with...suction cups from his Nerf gun. One beat later they explode. (No, not the suction cups...)
During the Cold War it was often alleged in the U.S. press that the Soviet Union built landmines to look like toys on purpose, especially in Afghanistan, though photographs hardly ever accompanied such allegations.
Soviet press, on the other hand was busy blaming Americans (who supported the local rebels) for the same. The most probable reason was that the antipersonnel mines widely used by the both sides were, first, virtually indistinguishable from each other, and, second, designed to be airdropped, so they had plastic wings to aid dispersal, which made them look vaguely as a toy butterfly for unsuspecting and uneducated eye.
How does an Afghan child tell the difference between a cluster bomb and a humanitarian food pack? It's not always easy, since both are bright yellow, about the same size and labeled in a language — English — that few Afghans understand.
To quote Robin Williams, "Now you're playing Survivor, the real game."
"The Pentagon announced last week that it will change the color of air-dropped food parcels so they won't be mistaken for unexploded cluster bombs."
Then there are the booby trap bombs, which are intentionally made to look harmless/fun to play around with. Popular designs include letter envelopes, food cans, and landmines made to look like bathroom scales (the design was quite popular in Vietnam, apparently)
Lawn darts. These foot-long mini-javelins with fins were supposed to be lobbed into the air in an attempt to hit target circles on laid on the ground. This was much like playing horseshoes with throwing knives, since the metal-tipped toys were essentially identical to Irish war darts. After forty or more years on the market, they were finally discontinued in the 1990s because of the obvious safety issues (and a string of hundreds of injuries per year).
Older sets are still out there, of course, and other countries might not ban them (of course, many other countries might have had the sense not to make the damn things in the first place). Also, you can buy the parts legally in the U.S. and still assemble them yourself. There are also all-plastic models which can be sold in both countries - still hurt if you throw them at people, but no more likely to cause bleeding than any other weighted plastic toy you can buy legally.
Human nature also plays a part. One Cracked.com article also points out that lawn darts are almost completely safe-if you play by the rules. It goes on to point out (needlessly if you know anything about children) that few were the children who were actually liable to do so for long, and it was only a matter of time before some bright spark decided to try catching it with their teeth.
Jeff Foxworthy included lawn darts in his set about dangerous toys that aren't allowed anymore. He also mentioned B.B. guns, wood-burning kits, and chemistry sets.
Foxworthy: [Lawn darts] didn't come with instructions, they just came in a box of eight! We used to take them out and just throw them straight up in the air! You catch one of those with your head you're getting coloring books for Christmas the rest of your life.
A horrific example: In 1999, 39 people were literally cremated in one of the worst tunnel fires in history in the Mont Blanc tunnel in France. The main fuel for the fire? Ordinary margarine. If you get the stuff to ignite, it burns as hot as diesel fuel; vegetable oil, one of its key ingredients, can actually be mixed in with diesel and run an engine.
Not helped by the truck's other cargo, flour, which as mentioned just below, is quite explosive when aerosolized.
In 2013, Norway had a major tunnel closed for weeks after 27 tons of caramelized brown goat cheese burned for five days. No injuries were reported. Still trying to get out the burnt cheese and fondue smell.
The Great Molasses Tidal Wave in Boston in 1919.
In that vein, the London Beer Flood of 1814.
The Banbury Custard Explosion of 1981.
No, they're not making any of these up. Though thankfully in the custard explosion, no one was killed. The same can't be said for the previous two.
In a similar vein, aerosolized flour is a huge explosion hazard.
A surprising range of otherwise innocuous substances are fire or explosive hazards when reduced to a fine powder. Coal dust is a major concern for miners for this reason, and powdered aluminum is used in certain explosives.
Mythbusters used non-dairy powdered creamer in an attempt to amp up the flour burning cannon design they replicated from a web video. They later confessed that the insane fireball they created made them fear for their lives.
Any kind of organically based dust—flour, sugar, coal, powdered wood, starch, etcetera—is highly explosive when suspended in the air.