Nautical Knockout

That's why they call it the "Boom"!

On a small, single-masted ship, the sail will be large and the deck so small you have to watch for the boom—it's the spar under the sail. So when the sail moves, it's likely to swing suddenly across the deck and knock the sailor unaware in the head. Sometimes this results in unconsciousness. Sometimes the sailor is knocked into the water. And sometimes, the sailor is knocked unconscious into the water.

In some cases, the target of the boom will get a warning that it's coming... but usually the warning comes a mere second before the impact, doing the target no good whatsoever.

Very much Truth in Television. When the boat is jibing (changing the sail orientation by moving the stern into the wind, as opposed to tacking, turning the bow into the wind), the boom can very suddenly swing across the deck at high speed, which can lead to a concussion (or worse) for anyone not paying attention. Keeping your head down is one of the first things taught to beginning sailors.

Compare Plank Gag.

Film

 * Cheaper By the Dozen - Happens to the father.
 * One Crazy Summer.
 * The Pirates of Penzance (the 1983 film) - The Pirate King runs into the boom during the Pirate King number (but it doesn't knock him overboard, though he does that later on his own).
 * Pirates of the Caribbean: Curse of the Black Pearl - Jack Sparrow does this intentionally to Will Turner.
 * Sabrina - Happens to Linus Laraby.
 * Joe Versus the Volcano - Happens to Patricia (Meg Ryan's third role) during a storm, knocking her out. Joe jumps in to save her, hero that he is.
 * Done deliberately at the end of Wild Things. The victim is then left to drown.
 * This is how the Leviathan actually kills the Viking captain in an infamous deleted opening for Atlantis the Lost Empire.
 * Played for laughs in the John Candy movie Summer Rental.

Literature

 * In the novel Man of the Century, 15-year-old Woodrow Lowe stows away aboard the yacht belonging to the prominent Balfour family, to try and get close to his love, their daughter, Amy. Almost as soon as he tries to venture on deck, he is hit smack in the face with the boom, which knocks him clean off his feet and right into the harbor.
 * In Kurt Vonnegut's novel God Bless You, Mr. Rosewater, Eliot Rosewater accidentally kills his mother when he takes her sailing on his small boat and tacks. The boom swung across, knocked her off the boat where she sank like a stone.
 * Subverted in Dragondrums, in that Sebell, nautically inexperienced, doesn't react when warned to get out of the way of the boom. Fortunately for him, Menolly yanks him out of the way just in time.

Television

 * CSI: Miami. Played For Drama in an episode where a woman died as a result of getting struck with a boom, rupturing her breast implants. No, really.
 * Doctor Who, "The Curse of the Black Spot" - Happens to Rory.
 * Happens to Pierce in Community while in a boating class. Only since the college is nowhere near water, the boat is in the middle of the parking lot, yet the instructor insists that they act as if they were at sea, and Pierce is essentially left to die.

Web Original

 * There is a minor piece of Polish internet lore which is a humorous dictionary of nautical terms. The dictionary mentions that the boom's main purpose is to sweep landlubbers, annoying crewmembers and miscellaneous litter from the deck.

Western Animation

 * The Simpsons - Treehouse of Horror XXI - It's how they kill Dr. House.