Cuteness Overload is the state in which a character experiences the immense rush of emotion after seeing (or hearing) something very, verycute, often rendering him/her partly or wholly incapacitated. The sudden charge of feelings could create so effective an incapacitation that the character may have to be woken up by another character calling his/her name or by other means. Depending on the situation, it shares some traits with a Heroic BSOD, but the main difference is an exposure to cuteness rather than a traumatic event.
Not to be confused with Cuteness Proximity (in fact, that one functions more as a subtrope), since this describes the state the character is in, whereas Cuteness Proximity requires the character suffering from the effects of Cuteness Overload to be within a particular distance from the cute thing — and often the cute thing in question must be a physical being or object. However, the former can obviously result in the latter, and the two tropes often do overlap. It may also result in I'm Taking Her Home with Me!
Most certainly Truth in Television.
Sakaki of Azumanga Daioh frequently experiences this with imagined cuteness, often involving cats, ("Kitty paddle..." as in the cat version of dog paddling), Chiyo ("Ribbons..."), or a combination of the two. The sight of her holding her head and jittering occasionally elicits concern from Osaka... or alarm from Kaorin.
Imagined Chiyo [in a cat costume]: I'm a kitty cat!
PENGUIN CHIYO. She will cause this reaction to occur in anyone who sees her, except Osaka.
In the Alabasta arc of One Piece, Nami and Vivi nearly experienced this when they saw the way the Kung Fu Dugongs were cuddling Luffy.
Invoked by a wild Teddiursa in the Pokémon animé, in order to be able to steal food from trainers and pin the blame on the other Pokémon. Once it evolves into Ursaring, the protagonists let it go, under the assumption that its tricks will no longer work. Once it runs into Team Rocket, however, it still attempts to look cute to ally itself with them. The effort met with confusion by the trio, but they let it join them, anyway, rationalising that such a powerful Pokémon would be a useful asset.
In Excel Saga, the Puuchuus are darling little teddy-bear critters; their main strength is their ability to enchant nearly everyone with their cuteness. Too bad they go severely Gonk when injured ...
In Kyon: Big Damn Hero, the first time Kyon called Mikuru by her first name he "completely blew past her threshold for withstanding cuteness." She couldn't help herself from kissing Kyon while saying it's forbidden at the same time.
Also happens to Haruhi when Kanae first calls her Suzumiya-sempai.
Mikuru: Eh? Suzumiya-san? Are you alright? You look dazed!
Haruhi (in a faint voice): S...something amazing happened.
The Hill Of Swords: When Sylphid hits people with her Moe even Emiya Shirou, the hardened veteran of countless conflicts who went through seven different kinds of hell, ends up being blown away by her cuteness. She once incapacitated an entire room with it.
Live Action Television
In one episode of The State, a skit portraying a grandmother taking her grandson clothes-shopping has this happen. As her overly-adorable descendant models his new suit for her, she flips out (borderline orgasmic-like) and asks the merch-worker who she had been chatting with, "Is there anything cuter than that?!?!" The worker proceeds to push her cute-button up to 11, with a step-by-step escalation, adding items to the boy's ensemble such as a bow-tie, puppy-dog earmuffs, and a little piggy-tail. When the grandmother inevitably reaches her threshold and goes violently berserk on her grandson (hey, it was the 90s), the worker and the store manager are forced to lure her into a bear-trap (a room containing an "emergency bear" that rips the lady to shreds off-camera), in order to "save the child". The worker blames himself for her death, while the manager blames the kid and abruptly pimp-slaps him for being "as cute as 2 little hamsters riding around in a little toy car."
In Noel's CT gag reel, the Cuteness Proximity of Litchi's panda makes her agree to act as Litchi's personal mannequin. To be fair, Litchi was also drunk.
Even though they never meet in CS Story mode (outside of gag reels), Ragna succumbs to this trope after downing Makoto, having trouble putting his attraction to her tail into words. To show this isn't one-way, she chides him for it if he's defeated. This trend seems to continue into CP, though...
"Tsubaki and Noel are the only ones allowed to touch my tail. Now shoo! Shoo I tell you!"
Gunnerkrigg Court, during Annie's visit to Foley House. She suddenly found herself in the center of a cuddlepile formed from the whole class of cheering ex-fairies, even more cute to the ether sight. And looked a little tipsy for some time after this treatment.
This happens in chapter 4 of Love Aquarium when Jwihyung sees a baby penguin all on its own. He doesn't notice the other penguins sneaking up.