Shattered Sanity

A person's mind is something that can be quite fragile, particularly if the person happens to be a cutie in a Crapsack World. The iconic fragile thing in fiction seems to be glass - very shiny and very fragile. This leads to it and its shattering being used as a metaphor for an individual's broken sanity.

A popular variant of this is the House of Broken Mirrors.

Anime and Manga

 * Occurs during Asuka's Mind Rape in Neon Genesis Evangelion at the end of a repeated montage.
 * Played with in Yu-Gi-Oh!, where the Other Yugi shatters Kaiba's mind after defeating him at the end of the Death-T arc. In this case, it's intended to give Kaiba a chance to regain his sanity as he puts the pieces of his mind back together.

Literature

 * One very well-known literary usage is in The Glass Menagerie, where the glass unicorn's horn breaking off symbolizes.

Film

 * One of the most frequent images in Sucker Punch is a glass breaking, once just before Babydoll's sister dies (From a light source) and the second time when Blondie dies.

Web Comics

 * In Narbonic, 's glasses crack near the end, symbolizing   descent into madness.

Western Animation

 * Avatar: The Last Airbender: Her real Jerkass Woobie persona is revealed in full at this point.

Real Life

 * King Charles VI of France (1368-1422) suffered from recurring bouts of insanity, including instances when he thought that he was made of glass, and this caused him to protect himself in various ways so that he would not break (Pope Pius II, who was born in the middle of Charles' reign, wrote of this in his Commentaries). Other people, generally educated and/or wealthy, were documented to believe in this glass delusion in Western Europe during 15th, 16th and 17th centuries.