Grandparental Obliviousness

"Homer: You're a useless old man. Name one thing you do for this family!

Grandpa: I watch the baby.

Marge: Where is the baby?

Grandpa: You left me with a baby?!"

- The Simpsons

A kid gets away with audacious stuff because they've been left in the care of a grandparent who is too senile and addled to notice if, say, they disappear through a Portal Picture to a Magical Land for a few days (probably because they're a victim of the Senior Sleep Cycle). Makes you wonder what in the world prompted their parents to leave them in said grandparent's care in the first place.

Anime and Manga

 * Mayumi Kinniku from Ultimate Muscle (less so in the original, where he's just Kinnikuman's dad)
 * The grandmother from the Suzuki alien arc in Gantz -- clearly not the sharpest knife in the drawer.

Film
"Grandpa (After killing the Head Vampire and going to grab a root beer from the fridge): "One thing about living in Santa Carla I never could stomach: all the damn vampires.""
 * Amanda Bynes' character in Big Fat Liar goes on an adventure with Frankie Muniz's character even though she is supposed to be staying with her grandmother for the summer. How does she get away with that? "That woman doesn't even know what year it is."
 * Combined with Parental Abandonment in Bad Santa: The Kid's mother is dead, his dad is in jail and he's left with his incredibly senile grandmother.
 * Grandpa in The Lost Boys turns the trope on its head; seeming clueless and, frankly, a bit weird, throughout the film, he eventually saves the day like he does it every day, handwaving the existence of vampires and completely stunning the other characters.

Live Action TV

 * In an episode of iCarly, Carly can't stop Nevel from messing with her website because while his mom is on a cruise, the only person in charge of him is his grandmother, "and she's hopelessly confused."
 * Subverted with Carly and Spencer's grandfather, who seems to be the sanest member of their family and actually shows in one episode to take Carly away from Spencer because of a perceived lack in responsibility from Spencer.
 * Subverted in Hannah Montana, when Miley's and Jackson's plans to party all night once their dad is out of town (their mother's been dead for a while) get frustrated by anything-but-oblivious Meemaw's surprise visit.

Music

 * The quest in Peter and The Wolf is done while Grandpa's asleep and oblivious of his grandson's disobedience.

Western Animation
"Homer: We leave you the kids for three hours and the government takes them away?
 * Lou Pickles on Rugrats
 * Although Tommy's parents aren't much better.
 * Any time Abe Simpson is left to babysit on The Simpsons
 * He's not much better with the cat and dog either, asking them which one of them was the mailman.
 * In the episode the Flanders adopt Bart, Lisa, and Maggie.

Grandpa: Oh, bitch, bitch, bitch."


 * But he subverts it on one occasion when he uses subtle emotional blackmail to get Bart and Lisa clear up all the mess they've made.
 * Abuelito in Mucha Lucha.
 * Timmy's Pappy on Fairly Oddparents subverts the trope. He cheerfully went along when Timmy, Cosmo and Wanda dragged him into an "old timey cartoon". Timmy got through it safely thanks to Pappy and the fairies, but when Pappy went on to describe it, Mom and Dad decided he was senile and hallucinating.
 * Max Tennyson of Ben 10 also subverts the trope. He knows full well what Ben is up to, and encourages Ben to be responsible about "going hero." Ben still gets away with audacious stuff, though, that would get a normal kid spanked or grounded.
 * Or dissected.
 * A variation on this trope existed in the Spider-Man and His Amazing Friends cartoon, in which college-aged Peter Parker's dear, sweet, clueless Aunt May rented rooms to Firestar and Iceman (of the X-Men). Not only did she have no idea that her tenants and nephew all had supernatural powers, but she never noticed that they tricked out the house in such a way that would convert the living room into a sort of above-ground Batcave when they pulled on a wall sconce. Even more amazing, they did all these technological renovations while she was napping one afternoon.

Real Life

 * Unfortunately.
 * Often averted in non-Western cultures or non-traditional family structures where grandparents play a major role in the raising of a child.