Dangerous Windows

In the horror genre, windows are not the safest thing. They're mostly thin breakable glass, often big enough to let monsters and crazed madmen use it as an improvised entrance and make for a nice spot to grab unsuspecting victims to their doom. Barricading windows can be hard work if there are too many of them and more often than not whatever is trying to get through, will.

Compare Super Window Jump, Destination Defenestration and Window Pain, in which windows are used for for improvised entrances, deliveries and exits, rather than monsters using them.

Comic Books

 * In The Seven Crystal Balls, two of the members of the Sanders-Hardiman expedition fall victim to crystal balls hurled through their windows. Professor Cantonneau is attacked this way just as Tintin tries to warn him not to go near the window. Dr. Midge is then placed under police guard, but The Guards Must Be Crazy and turn away from the window just long enough for another glass-shattering attack to happen.

Film

 * Shaun of the Dead. Most notably when foolishly stands with his back right against a window, and (predictably, considering his recent actions) the zombies break through and pull him out to eat him.
 * Tremors 2, several times.
 * In the original A Nightmare on Elm Street,
 * The Friday the 13 th movie series loves this.
 * The very last scare of Part 2 was Jason busting through the window in slow motion in a hallucination the Final Girl had.
 * In In the Mouth of Madness, a lunatic crashes through a cafe window with an axe in an attempt to kill Sam Neill's character.
 * Subverted in Independence Day: the monster smashes into the window but it doesn't break.
 * And then promptly Inverted.
 * "Is that glass Bullet Proof?"
 * "No Sir!"
 * *cue the dakka*
 * In The Descent, Sarah gets impaled in the head by a steel beam crashing through the window. But don't worry, it was All Just a Dream.
 * The killer in The Toolbox Murders gets to do this.
 * Feast
 * 1408 has way too much fun with this trope.
 * The Fog (1980). Zombies break through stained glass windows in a church to attack the protagonists.
 * In the original Romero Night of the Living Dead the hero is trying desperately to board them up while zombies reach through the broken glass for him. Great tension and suspense builder.
 * Dreamscape. While Alex and the boy Buddy are in the house, the snake man breaks in through a window and attacks Buddy.
 * Lampshaded in The Mist; everyone is inside a supermarket, and once they realize that there actually is something God-awfully horrible out there, someone points out that the entire front side of the building is a row of windows. The next few minutes consists of the hold-outs piling up everything they can to block the windows off, with only a relatively small bit of the tops exposed.

Live Action TV

 * In the Buffy the Vampire Slayer episode where they throw a coming-home party for Buffy, they end up getting attacked by various monsters (of course), many of whom attack through the windows.
 * After Xander acquires carpentry skills, he lampshades the frequency of this occurrence, saying he wonders why he even bothers fixing the frames.

Video Games

 * Resident Evil, and Capcom horror games in general have things breaking in from windows.
 * This is the method used by the various enemies to enter in the Nazi Zombies mode of Call of Duty World at War. Barricading them (and their inevitable being broken down again shortly after) is an important game mechanic.
 * In the Action 52 game City of Doom, touching windows kills you instantly.
 * In one level of the horror-shooter Metro 2033, there is a incredibly tense sequence where you are walking through a ruined library on the surface and are ambushed and trapped within a small room by one of the strongest monsters in the game, the Librarian. When you try and break down a rotting door to get out, the thing attacks.

Web Comics

 * An early Penny Arcade strip features the line "'Survival Horror' is a fancy way of saying 'Monsters will come through windows'."