Nobody Ever Complained Before

This trope is when one person or group has a habit or custom that they perform pretty regularly. The custom is questionable in some manner. Perhaps it's aggressive or off-putting. Perhaps it's downright ugly and offensive. Either way, no one has ever objected in any way before, either to merely say something or to actually put a stop to it.

When someone finally does act or speak up, the objection comes as a source of puzzlement, confusion, and sometimes outright anger on the part of the person or group! Someone is objecting? Why would they object? How dare they object!

A Sub-Trope of Idiot Plot.

Examples of Nobody Ever Complained Before include:

Films -- Live-Action

In Erik the Viking, the soldiers of Evil Overlord Halfdan the Black are so surprised that Erik is actually fighting back that they don't know how to react to him. People are usually too scared of them to try fighting back at all. This encourages the rest of Erik's crew to jump in and fight.
Scary Movie 3: The aliens appear to attack the protagonists, who then kill one of them in retaliation, but the aliens inform them that strangling each other is their standard way of saying hello. A kick to the groin is how they say goodbye.
Waterworld: The Deacon prefers to get information by capturing two prisoners and telling them, "Whoever talks first gets to live," then shooting the one who didn't talk fast enough on the spot. This apparently works so well that, when he's captured both Helen and the Mariner and neither of them talk, he has no idea what to do.

Live-Action TV

In Babylon 5, the Minbari routinely salute other Minbari vessels by approaching with gunports open as a sign of respect. When they encounter a human vessel for the first time, they attempt to do the same as a friendly gesture. The Earth ship, utterly misinterpreting this maneuver and convinced the larger alien vessel is about to attack, opens fire first. The Minbari leader Dukhat was enraged when he learned his crew was doing this in a first-contact situation, but he was killed in the process of ordering the gun ports closed. A bloody war immediately followed.
In several Star Trek episodes, various alien species actually seem surprised when the Federation objects to having their citizens kidnapped.
- In the episode "Half a Life" there's an entire species of people who ritualistically kill themselves on their 60th birthdays, and they seem shocked and baffled when one of their own refuses to do so so (because he needs more time in order save the whole planet—also, he'd fallen in love with Lwaxana). Apparently none of their 60-year-olds had ever had any qualms about dying before.
On Seinfeld Elaine's dancing is truly horrible, but no-one can ever bring themselves to tell her... until Kramer sees her dance, that is.

Literature

We can only assume that the piggies of Speaker for the Dead had never tried to disembowel another species before, or else they might have some idea why the humans objected.
- Justified, between when the practice started and when the humans showed up, no other sentient species existed on their planet—there were only 6 other species on the whole planet. Forget no other sentient species, there were barely any other species, period!
In Harry Turtledove's Worldwar series, members of the Race cannot understand why humans insist on fighting back instead of just placidly accepting their rule. Don't the humans realize that the Race is by definition the most superior species in the universe and thus natural rulers of all life? How dare they think they can resist! And how dare they actually win!
In The Conquerors Trilogy by Timothy Zahn, first contact with the Zhirrzh goes very badly because the ghosts they use for communication (It Makes Sense in Context) are fatally sensitive to radio waves. Humans are the FOURTH species this has happened with (the other three they conquered, thus the trilogy title). They never figured out that these species were trying to communicate; instead, clearly, all those other species attacked them at first contact. And those other species were so barbaric they armed their escape pods! And so on.
Played for horror in Shirley Jackson's The Lottery: The premise is a town that holds an annual lottery, where the "winner" of the lottery gets stoned to death by the rest of the locals. The townsfolk have no qualms about this, even going as far as a mother getting stoned by her own children when she receives the wining ticket. Other towns have begun to abolish this lottery in protest, along with one person in this town, which the other townsfolk can only interpret as societal regression.
In The Ringworld Engineers, sex is often used as an interspecies social/political lubricant. When Louis Wu turns down offers, he is usually greeted with confusion or amusement.

Western Animation

The Simpsons: Homer has gotten Apu fired from the Kwik-E-Mart.

Apu: It is time to settle the score!
(he walks up to the door and knocks; Homer answers it, and Apu walks slowly towards him, arms outstretched, an angry scowl on his features; Homer walks backwards, and screams in fright)
Homer: No, don't kill me. I didn't know there was film in that camera in that hat! I was unaware. I was unaware!
Apu: Mr. Simpson, you misunderstand me. In my village this is the traditional pose of apology... You know, now that I think about it, it may be a little confusing. Many have died needlessly.

American Dad: When Stan goes to Heaven and is denied a chance to return to life, he pulls out his gun and threatens to shoot. Everyone laughs and points out that Earth Guns don't work. The Bailiff pulls out a Heaven Gun and mentions that these do. Stan immediately grabs the gun and threatens to shoot his now-hostage lawyer. Everyone is shocked and stunned while Stan runs away. One guy complains.

(Stan grabs the gun; various reactions of gasping and other comments from the crowd)
Bystander: Why do we have those again?
(Stan runs away with his lawyer as a hostage)
(Beat)
Bystander: Seriously, why do we have Heaven Guns? I don't mean to be that guy, I'm happy here... but why is this not an issue?

Real Life

The Aztecs knew how to besiege others, but they had no experience surviving under siege themselves when the Spaniards did it. That was just one of many factors leading to their fall.
Every political or cultural revolution, ever. The people rise up against an inherent inequity in law or society that everyone just simply accepted—or at least never really thought about—until now.