A bafflingly dark M&Ms ad had a man realizing that his girlfriend never opened the gift that he had gotten for her, and after rushing to the closet and tearing it open he watches the anthropomorphic M&M inside chokes on its last breath.
There was an incompetent Mad Scientist Dr. Fishtein in Gorsky and Butch. All his death traps involved some kind of predatory fish, always dead because he kept forgetting about water...
Averted in the miniseries Thessaly Witch For Hire. The title witch is revealed to have captured or enslaved several monsters over the centuries. When her would-be suitor Fetch accidentally makes her a target for a seemingly invulnerable beast, she decides to let most of these monsters go because she doesn't expect to survive her encounter with the beast, and thinks it would be cruel to let her assorted captives die.
In one Daredevil story arc, the hero ends up in an old mansion converted into a gigantic house of DEATH. At one point, he gets thrown into a tube and ends up in a pool... which, due to lack of maintenance, featured a half empty base, and a suffocating shark.
The comic book adaptation of Indiana Jones and the Fate of Atlantis features this (somewhat surprisingly, given the franchise's general use of Durable Deathtrap)— Indy and Sophia fall into a pit of alligator skeletons, prompting Sophia to remark that it isn't really much of a deathtrap anymore.
In Knights of the Dinner Table, this serves as the basis for the classic "Bag Wars" arc. Brian, while checking the list of what his character is keeping in his bag of holding, discovers a complement of hirelings he had placed in there and forgotten about. When he investigates, he discovers that the hirelings have used the resources of the bag to build a keep and declare themeselves an independent nation. Writer Jolly Blackburn claims this storyline was inspired by an actual incident in one of his games where a player hid hirelings in a bag of holding in order to smuggle them into an enemy stronghold and then forgot about them.
In Wormy, the "Siege of the Iron Keep" wargaming set that Wormy had ordered took four months to be delivered. Unfortunately, the enclosed playing pieces had only been provided with six weeks of rations.
In the Discworld book Wintersmith, Roland and the Nac Mac Feegle travel to a disused Underworld to rescue the Summer Lady, and find the skeleton of a three-headed dog that had apparently starved to death.
In Sourcery, there's a sequence where the Grand Vizier attempts to find a way of killing/torturing Rincewind, but keeps being told there's something wrong with the device. For example, when he suggests throwing Rincewind in a cage with a tiger, he's told the tiger is ill.
Subverted in the H.P. Lovecraft story The Dunwich Horror, where the monster escapes from its prison when its brother, Wilbur Whateley, fails to return to feed it after being mauled to death by a guard dog.
In a variant, one of the Magic: The Gathering tie-in novels had an ancient (and slightly senile) wizard summon up a vicious unnatural horror he remembered from his youth - only to get a heap of dusty bones. This prompted him to think something along the lines of "Has it really been so long?"
Played straight in the fifth season of Angel, when Illyria returns to Vahla'hanesh to get her demonic army. She's been dead so long they have all crumbled to dust.
One episode of Horrible Histories covered Tudor medical treatments. One of them involved covering live spiders in butter and feeding them to the sick victim. They were about to try it, but couldn't, because they forgot to feed the spiders.
One woman on an episode of Fatal Attractions was attacked by her pet tigers because she could no longer afford to feed all of them and had to resort to scrounging for roadkill.
In The Big Bang Theory, Howard is preparing to do a magic show when he finds an old trick in his closet involving a metal tin and a live dove. When he opens the tin, feathers fly out and the sight makes him retch.
Keeping your pet(s) fed in NetHack is important (and fortunately quite easy). Feeding them treats will train them to fetch. Also, being separated from your pet(s) (on a different level) for too long will lead to them going feral. A possible YASD is being killed by a former pet.
Averted in Nintendogs: If you neglect to feed your dog, eventually it will simply "run away." But it returns no worse for wear (if a little dirtier) soon afterwards.
Dogs and cats in the games are treated like infants/children or infants, toddlers, and children depending on what version you are playing (apparently Child Services doesn't care about teens), but fish and other small animals can be starved if you forget to feed them.
In The Sims Medieval YOU can forget to feed the monster! Specifically, there's a huge tentacled beast used to execute Sims, and Spies and Knights are frequently assigned "Feed The Beast" as a responsibility. Sadly, they can only throw meat in, not Sims. And if they do literally forget to do it, the monster doesn't seem to suffer any ill effects. On the other hand, there is a quest where you have to cure the Beast of an illness.
As of 2012, herbivorous animals in Dwarf Fortress need to graze in a pasture, or they will starve to death. The mechanics of this are still pretty wonky, and several larger animals literally cannot eat fast enough to keep from starving. So much for your army of war elephants.
In Mass Effect 2, the captain's cabin on the Normandy 2 features a huge aquarium that you can fill with fish. And if you forget to feed them between missions, you'll be refilling it with fish, over and over and over again.
This problem is cured in Mass Effect 3, wherein you're able to purchase an automatic fish feeder for your aquarium.
O tries to invoke this in a Commissioned strip, to deal with a player who keeps his familiar in a sack and never makes any reference to it except when it's currently useful. He's talked down to the familiar having just run away due to neglect.
Also used dramatically in The Dementia Of Magic: When the hero sees all sorts of dying monsters in the dungeon, he correctly guesses the evil sorceress that has been harassing and taunting him is not that competent.
Liz: Wow, I forgot I had this fairy in a bottle thing.
Subverted in The Order of the Stick, as Evil Overlord Xykon responds to his minions' complaints of not being paid/fed/whatever by killing them and raising them as zombies. Because they're just as strong, and cheaper to feed.
Inverted in Sluggy Freelance, where failure to give Satan's feline offspring milk on a daily basis results in them becoming extremely dangerous, and they're otherwise docile. The entire plot of the first K I T T E N all started because one of their caretakers switched from the real deal to a soy-based substitute; the sequel involved a government conspiracy depriving the town of milk so they could reproduce the conditions of two years ago.
In one instance, Peter reveals he'd been saving a pony for an easy way to cheer up Meg when needed. Opens the closet, sees its skeleton.
Peter: Uh that's right, ponies... ponies like food, don't they?
A similar thing occurrs in another episode where he stashed party balloons, streamers, and a clown on the chance where he was finally right about something.
Lois: You were right, Peter. Peter: No way! I finally get to do this! (pulls on a rope which drops balloons and confetti and unrolls a banner that says Peter's Right) Peter: I had that set up fifteen years ago. Hey where's the clown? Lois: We have to do something about the FCC. Pack your bags, Peter, we're going to Washington DC. (skeleton with a clown nose and a rainbow afro wig falls from the ceiling) Peter: Oh, there he is.
In an episode of Garfield and Friends, Jon digs a top hat out of a trunk while mentioning that he hasn't practiced the trick in 20 years. He then reaches into the hat and pulls out a rabbit skeleton, to which Garfield remarks, "Maybe you should've fed the rabbit."
The Monarch had a trap in his lair which consisted of releasing a horde of "man-eating butterflies"... except he forgot to feed them, so Brock just had several hundred dead butterflies dumped on his head. The Monarch then berates his henchmen for the mistake.
On a related note, at one point Dr. Venture is going through his mail, and opens a package containing a coiled cobra, poised to strike, and the words "Die, Dr. Venture!" written in blood. After a moment, the snake crumbles into dust, at which point he checks the postmark, realizing a) the package has been on his desk for years, and b) "I really have to get my shit together."
Neglect has led to the death of many a beloved pet (or even child) in Real Life, for reasons ranging from forgetfulness to addiction to malice. Since this is neither funny nor relevant, please refrain from adding examples.
There's a classic riddle about a man who is given the choice of taking his chances with cannibals, or being thrown into a pit full of lions who haven't eaten in a year. He chooses the lions and escapes, because the lions have obviously died at this point.