Plank Gag

Train Conductor: "Board! Board!"
Passenger: *Looks at watch, sees that it's not nearly time to leave, scoffs*
(The passenger then gets hit by a passing by man with some wooden boards, twice.)
Train Conductor: "Told ya. Told ya twice."

—Hot Shots

A subtrope of Slapstick and Amusing Injuries, this is an old gag where Alice is carrying long items such as planks, logs, poles, or ladders, turns around and accidentally hits Bob, who was standing behind or next to her.

This happens either because Bob called Alice from behind, and she instinctively turned around to greet him, or because she wasn't aware of his's presence, or simply because Alice isn't careful of her surroundings.

One common variant is to have the carrying character unknowingly knock out an attacker that was furtively approaching from behind. Another is to have a sufficiently quick-witted character duck the first swing, only to be clocked in the head when Alice turns back around.

This gag find its origins in Vaudeville and Music Hall, leading to its use in the Silent Movie era. Virtually every silent movie, starting from Chaplin to Laurel and Hardy, used this gag at least once, even when talkies came along. The Plank Gag was a favourite of the Three Stooges and the Marx Brothers. It's still used on stage today in the UK with Pantomime.

In Real Life, safety rules are set up specifically to avoid this situation.

Characters likely (but not necessary) to be involved in the gag are The Klutz for the long item-wielder, and the Butt Monkey and The Chew Toy for the person suffering the hit.

If you're thinking about a guy getting hit in the face by a board that he stepped on, that's actually a variation of the Rake Take. Also compare Nautical Knockout. A Plank Gag might be set up, but subverted by the Coincidental Dodge.

Examples of Plank Gag include:

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An old Australian television ad for insurance had some average Joe walking along the street, narrowly avoiding accidents. One of the accidents he ducks (literally) is the Plank Gag. He drops the coin he is flipping and bends down to pick it up just as the workman with the plank turns around, the plank passing harmlessly over the top of him.

Anime and Manga

In Zoids: New Century Zero, this happens when Bit unintentionally knocks out several thugs with a large pipe. He's confused as to why they are unconscious afterwards.

Comic Books

The Gaston Lagaffe books made use of this gag several times, with the eponymous klutzy character obviously being the one accidentally smashing the items on his unfortunate working colleagues' faces.
Happens once in the Bone collection the Great Cow Race. One page has Smiley holding a ladder from one end while the other is between Lucius and Phoney. In the back is Fone Bone is calling to Smiley, pointing to the left. In the next page, Smiley has completely turned around, while Lucius and Phoney are knocked over, clutching their heads in pain. Fone Bone has an "Oops!" look on his face.

Film

In My Fair Lady, happens during the "With A Little Bit Of Luck" song, since Alfie Doolittle is singing in what seems to be a construction area and there is inevitably someone who swings a plank around and someone else gets hit by it.
Vintage British comedians Eric Sykes and Tommy Cooper actually made an entire 45 minute film devoted to exploring and using every possible permutation of this gag. It is simply called The Plank and is much better than it sounds.^[1]
In Seven Brides for Seven Brothers, one of the townsfolk "accidentally" does this to one of the brothers - although he's openly a Jerkass about it and he never really expected anyone to believe it was an accident.
In The Voyage of the Dawn Treader, Eustace is attempting to set an oar in a rowboat (backwards), nearly loses his balance, and swings it about, knocking out a pirate leader who was sneaking up behind him with a knife.
This gag was incredibly common in The Three Stooges.
In Star Wars: Return of the Jedi, Bobba Fett, about to shoot at Luke, gets accidentally thwarted that way by Han, and knocked off the skiff to the monster's mouth.
Occurs in Godzilla Millenium (Godzilla 2000 in the West) when Yuki visits Yuji and Io, and has to ask two dockworkers for directions to the Godzilla Prediction Network HQ.
Invoked intentionally in Down Periscope. While everybody else is doing scut labor repainting the sub's deck, Stepanek sets up a lounge chair and goofs off. (he's trying to get himself thrown out of the military, and The Captain knows it, but isn't willing to oblige). After getting Stepanek's hopes up with a promise of "relocation", the Captain barks at a passing sailor who is carrying an armload of planks. The sailor spins, swatting Stepanek off the deck, thus successfully relocating him into the sludge tray that was alongside.
Hot Shots Part Deux, as quoted above.

Literature

In The Pendragon Adventure, when Bobby completes his Training from Hell with Loor and Alder and is awarded a real fighting stave, he launches into a long series of these. He'd nearly whack one of them with the stave end as he turns to the other to apologize for nearly whacking them, and then spins around to apologize for that...
The Discworld book Making Money features a martial art based on slapstick routines, including carrying a ladder over one shoulder and hitting assailants with it when one turns around.
- This is also why the Fools' Guild's fire department has never managed to stop any fires; you hand a group of clowns a ladder and a bucket, and the instinct to pull this gag immediately takes over.

Live-Action TV

Chucklevision had one of these practically Once Per Episode, accompanying the brothers' Catch Phrase.
Happened with some regularity on Home Improvement, usually with Al on the receiving end. And once with Bob Vila.
Done with some frequency on The Benny Hill Show.
Monty Python Live at the Hollywood Bowl includes a "comedy tutorial" sketch featuring plank and other slapstick gags. The routine was originally written for the TV sketch show We Have Ways of Making You Laugh (1968).
- Notable for the sight of Terry Gilliam in obvious and considerable pain with the effort of trying not to bust up laughing.
In this scene from 3rd Rock from the Sun, it happens no less than five times.
A Mystery Science Theater 3000 skit had Tom Servo with a long canoe on his head, explaining the concept of "portage". As he turns his head this way and that, he repeatedly knocks out Crow and Mike.
- Another skit had Mike explaining the gag to Crow and Tom using an actual plank. While Mike was close to tears about the gag, the bots where...less than amused.
In the Doctor Who, story "Dragonfire" the bait and switch version of this gag is played. Where it looks like the Doctor is going to accidentally clobber Glitz (with the Doctor's umbrella in place of the plank), but Glitz is Distracted By the Shiny and ducks at the last minute unwittingly letting the umbrella swing harmlessly over his head.
The Star Trek: Voyager episode "Suvival Instinct", had Chakotay trying to lug a huge piece of alien sports equipment across the bridge and nearly whacking a visiting alien with it.
Prone to happen anytime El Chavo grabs a broomstick or similar in El Chavo del Ocho.

Professional Wrestling

A stock spot in Professional Wrestling ladder matches, or other hardcore matches with a ton of weapons. More commonly the ladder is used to assault other wrestlers but this spot is common as well, especially in team matches.

Puppet Shows

Magellan from Eurekas Castle has a Running Gag variant on this. His tail is the long object, so for instance he'll build something out of blocks and knock it down with his tail when he turns around. Typically he then scolds his tail as if it were sentient.

Theatre

This was demonstrated as part of the 'Comedy Masterclass' skits in The Secret Policeman's Ball.

Video Games

In Tokimeki Memorial 2 Substories: Leaping School Festival, this happens in an Event of Akane's storyline, when the protagonist, surprised and worried to see her carrying such heavy-looking logs (which are no problem for her, being from a family of martial artists), calls her from behind.
This is one of Gintoki's attacks in Jump Ultimate Stars. The attack might be based on chapter 9 of Gintama, but the actual gag doesn't happen there.

Western Animation

On The Penguins of Madagascar, King Julien challenges Fred the squirrel to a duel with staffs. Fred beats him without even trying, just by moving around with the staff on his shoulder.
In |Toy Story 2, Rex the green plastic Tyrannosaurus Rex accidentally knocks Evil Emperor Zurg off an elevator with his tail while shuddering in fear as Zurg is about to destroy the two Buzz Lightyears (one of them has a plastic belt around him).
In the Camp Lazlo episode "Snow Beans", this happens to Chip and Skip, using a pair of skis rather than a plank.
Axl does it to Bull Gator in an episode of Taz-Mania. Bull responds with a Genre Savvy "I really should have seen that coming".
Gunther does this to Kick on Kick Buttowski a few times in the appropriately named episode Knocked Out.

↑ also possibly the only film in existence with a Gainax Beginning

[1] so possibly the only film in existence with a Gainax Beginning

[1]