... a propensity, while in use, to turn instantly beastly hot or blindingly cold upon you, depending on whether your neighbor turned on his cold or his hot to deprive you of a necessary complement in the shower you had so carefully blended.
Bob flushes the toilet while Alice is in the shower. Alice screams and curses, unprepared for the sudden increase in water temperature. Or perhaps Alice turns on the washing machine while Bob is in the shower, causing Bob to experience a precipitous drop in water temperature. Can be done deliberately or by accident, but the result is still funny — provided it's not happening to you. Can also be done by turning on a faucet elsewhere in the house, which has the advantage of not prematurely giving away the game if you are deliberately pranking someone. Usually deployed as a comedy trope, often accompanied by a quick shot of the Oh, Crap expression on Alice's face when she hears the unmistakable sound of a toilet flushing.
Truth in Television as most shower valves mix hot and cold water to achieve the desired temperature so anything that diverts either hot or cold water will result in a sharp temperature shift in the opposite direction. Note that this was always a regional trope as it depends upon the central water heater practice common in Western plumbing design but not always found in other cultures. Rapidly becoming a bit of a Discredited Trope in Real Life since the introduction of pressure-balanced shower valves specifically designed to prevent this from happening. Such "anti-scald" shower valves are even required by law in some places, especially in commercial buildings like apartments, hotels, and dormitories. Nevertheless, if you have the (mis)fortune of being in a house or building with old plumbing, this trope is still doable.
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A Sensodyne advert features a woman turning on a tap. The man in the shower shrieks.
A Zoo Tycooncommercial shows this happening to a woman. Thing is, though, the one flushing the toilet is an elephant.
A Dutch commercial for the Army (Fit/Unfit) had two ten-year-old girls giggling as they turn on the faucet, one commenting after a bit "I don't hear anything". Cue a guy manfully enduring the cold as he soaps up and scoring a "Fit" in a multiple choice box. Watch here.
In Jersey Girl, Gertie is always needing to be reminded by her father to flush the toilet. This ends up being a Chekhov's Gun: Gertie comes home while Ollie is trying to be intimate with his friend Maya, so they hide in the shower, and Ollie pretends to be alone in it when Gertie comes in to use the toilet. This, of course, is the one time Gertie remembers on her own to flush, causing Maya to scream in pain and Gertie to catch them red-handed.
Howard: Why do you always find that so funny? George: It's very hard to say.
Katherine does it to Sebastian in Cruel Intentions 2. Only difference being he slips without screaming.
In the first Garfield live-action movie, Garfield flushes the toilet with the apparent intention of forcing Jon to exit the shower due to the increase in water temperature (he has, by Garfield's estimation, been in there too long).
Gina Davis' Hot Scoop character in The Fly. Her Jerkass publisher/ex-boyfriend breaks into her house and takes a shower while waiting for her.
Quint McHale: I'm going to take a shower. You know what that means? Roberto: Don't flush.
Dolores does this as a prank on Humbert in the 1997 film adaptation of Lolita.
Neal Stephenson's early work The Big U has one student getting revenge on his former roommate by getting a small army of friends and other acquaintances, synchronized by a TV news broadcast, to do a big flush that actually destroys the plumbing system in an entire wing of the arcology-university. Was also coupled with supergluing shut the door to the shower stall, and then setting a "poorly insulated" electrical appliance in front of the expanding pool of water.
Grace Under Fire: One of Grace's daughter's little friends is chatting with Grace, despite the fact that she's trying to shower in peace.
Boy: Want to hear a joke? Grace:(sighs) Sure. Boy: What did the man say when he fell off a cliff? Grace: I don't know; what? (boy flushes toilet; Grace screams) Boy: That's right! (runs away)
Nash Bridges: In "Sacraments", when the officer on SIU team Evan is having a romantic shower with Nash's daughter Cassidy, Nash's partner Joe drops by with info on SIU current primary investigation. Try as the pair might to keep their relationship secret for a while more, Joe has to flush a tissue of his allergy snot down the toilet. Cassidy yelps, jumps from behind Evan and rips a curtain off. All that's left is for her and Joe say "Hi" to each other.
In The Sims 2, if one Sim flushes the toilet while another is in the shower, the second Sim will jump out of the shower, run over to the first Sim and yell at them in Simlish. Of course, someone made a mod to disable this.
It is revealed in one of the Penny Arcade podcasts that for the entire time they lived together (a period of "at least a year"), Jerry (Tycho) would wake up early and turn on the taps to allow all the hot water in the apartment to run down the drain. The moment where Mike (Gabe) learns why he didn't have a hot shower during the whole time is priceless.
Mike: What the fuck is wrong with you!?!
The episode is for the strip "Doctor Feelgood". Strip.Podcast. Story starts at about 17:10, but they whole thing is really funny.
Season six's "Bart vs. Australia" — Bart is trying to get the toilet water to go counterclockwise and repeatedly flushes the toilet. Meanwhile, Homer is in the shower and keeps screaming "Hot-cold-hot-cold-Who's-doing-that?!"
Season twelve's "Tennis the Menace" — Bart turns on the kitchen faucet and Homer (once again in the shower) screams, and to tunes such as "Mary Had a Little Lamb".
Max (of "Sam &" fame) blindly reaches for a towel while singing in the shower and accidentally hits the lever on the office toilet, burning the hell out of himself while also inadvertently flushing a tiny assailant down the crapper.
Grandpa's hated, visiting sister does this to him in Hey Arnold!
In Phineas and Ferb episode "Backyard Aquarium", this was done with a hose in place of the toilet.
Shown as one of the many pranks Huey, Dewey, and Louie pulled on Donald that got them on Santa's naughty list in Mickey's Twice Upon a Christmas.
Done in one Robot Chicken sketch: a girl flushes the toilet, and her father is scalded.
Done in Channel Chasers, with Cosmo being in the shower and Wanda flushing the toilet. Fridge Logic hits when you remember that they are a magic shower and magic toilet, so, not interconnected (probably just Rule of Funny).
Slinkman from Camp Lazlo deliberately does this to Lumpus in "Slugfest".
Rallo does this to Cleveland Junior on The Cleveland Show. To add insult to injury, he takes a picture of Junior naked and presents it in a slide presentation, which causes Junior to get kicked out of the scouts.
In one episode of Cro, Phil the gets stuck in the shower. After all attempts to pry him out fail, Dr. C decides to flush the toilet, and the swift change in temperatures sends Phil flying from the shower stall.
It happens. With old water heaters, ill-timed laundry, or what have you.
In places with electric showers, a plumbing network that has the toilet and the shower in the same manifold (which is doable but considered incorrect for reasons that go farther than thermal comfort) will cause this, either because suddenly the shower's resistor is heating up less water at a time, or because the flow gets so low the heating is disabled altogether.