Fumbling the Gauntlet

(Mal punches Atherton)
Atherton: I accept.
Mal: That's great! What?
Warrick: (to the crowd in general) There has been a challenge.

—Firefly, episode "Shindig"

In an unfamiliar social environment, a character does something that he thinks is mostly innocuous, either intentionally or by accident. Unfortunately, in this society, said act constitutes Throwing Down the Gauntlet, and will result in him engaging in a Duel to the Death, possibly on the spot. Oddly enough, the people from the local culture never seem to grasp the concept that different cultures do things differently, so they never understand the poor foreigner's plight, and he does not get a free pass for not understanding the gravity of what he just did.

See also Accidental Marriage for other results of innocuous gestures. For a less Life-or-Death version, see Fee Fi Faux Pas.

Examples of Fumbling the Gauntlet include:

Anime and Manga

In Ranma ½, Girl-Ranma does this in Shampoo's village by eating the prize that Shampoo was to win. Shampoo challenges 'her'; Ranma defeats Shampoo, and winds up earning a Death Sentence.
In Kyou Kara Maou, Yuri accidentally proposes to Wolfgang by striking him across the left cheek. Wolfgang is so embarrassed and furious that he throws the tableware to the floor - resulting in Yuri accidentally accepting Wolfgang's duel challenge by picking up a knife.

Literature

In Terry Pratchett's Johnny Maxwell Trilogy, Johnny bumps into an alien. Trying to look harmless, he shows his empty hands and smiles. The alien returns the gesture... spreading his claws and showing off his teeth. Also, several of Pratchett's books contain a point of advice: Never grin at an orangutan.
The same happens in a Xanth story, by Piers Anthony. Dor meets a giant spider, and tries a "hug" gesture to show friendship. The spider thinks he is readying his fangs.
In the Starfire series by David Weber, the Orions consider a toothy smile a challenge. There is also an inversion where a person mistakes an Orion's toothy smile for a friendly gesture.
In Anne McCaffrey's Acorna Series, Acorna often unnerves her fellow Linyaari by smiling human-style (with teeth showing). Among her people (Acorna was raised by humans since she was a toddler), baring your teeth is a display of naked aggression, akin to walking around with your fists up.
In Robert A. Heinlein's The Moon Is a Harsh Mistress a teenage girl flirts with Stu, a tourist from Earth. He takes the initiative and tries to kiss her, not realizing that this is a capital offense. Partially justified, as the Loonies invoked this as part of their justification for their planned independence war.
One of the characters in Timeline does the opposite. An opponent throws down the gauntlet to show his contempt, but our hero apparently didn't know that picking it back up meant he accepted. Even though he was an archeology student recruited for his knowledge of history.
The Final War of the Bolo continuum was begun in part due to standard Melconian First Contact procedure: Enact a Non-Intercourse Edict until the governing body has come to a conclusion. The Concordiat saw this as a sign of aggression and the situation deteriorated from there.

Live-Action TV

The Firefly episode "Shindig": Mal hauls off and decks Atherton Wing after the latter hits the former's Berserk Button by implying that Inara is a whore (only Mal gets to call her that). Turns out that punching someone in a social function on that planet is considered a formal challenge, and Mal ends up having to fight a sword duel over her honor.
- Mal also manages the sister trope of an Accidental Marriage two episodes later. Either the 'verse has some surprising cultural diversity, or Mal is surprisingly sheltered. While in the first case, the rules of society escalated the physical conflict far more than Mal expected, in the case of his accidental marriage all Mal did was accept some wine and an offer to dance while attending a party.
The Babylon 5 episode "There All the Honor Lies" narrowly averts this when Sheridan accuses a Minbari of lying, but Delenn vehemently explains that the accusation would have called for an immediate and fatal response if the Minbari had been present.
- The Earth-Minbari War started because of this trope. The Minbari ships approached with gunports open; to the Minbari, this was a gesture of respect akin to the Johnny Maxwell example above. Unfortunately, it was taken to be a hostile gesture (weapons ready, we come in war), and the rest is very bloody history.
Star Trek: Voyager: One episode has Janeway insult an alien race by putting her hands on her hips, provoking a conflict due to the "obscene gesture".
Star Trek: Deep Space Nine: In episode "Apocalypse Rising", Sisko is pretending to be a Klingon, and as part of the effort, strikes Worf with the back of his hand. Worf then asks if he meant to challenge him to a battle to the death and suggests to simply strike him with his fist instead.

Video Games

In Shadowkeep, entering the titular fortress while wielding a weapon would cause the guards to treat you as hostile (i.e. kill you.)
In Karateka, don't approach Princess Mariko in fighting stance. Just don't.

Western Animation

Inverted in The Simpsons, when Homer goes around Throwing Down the Gauntlet not expecting it to be picked up.

Homer: Why oh why did I slap a man who says 'suh'?!

Storm Hawks at one point takes the Sky Knights to Junko's home, where Piper accidentally bumps into someone. She raises her hands placatingly - and Junko hastens to intervene, because this is how his people challenge each other to a fight.

Real Life

Japanese culture has a lot of non-verbal signs that can be easy to botch. But there was one even locals could fall for: in the era of the Samurai Culture, emphatically striking one's sheath with the sword's guard was a gesture of a challenge. Given that it's the culture which invented Iai-jutsu, this little tradition probably kept the number of warriors with loose sheaths rather low.
Chimpanzees and many other apes consider showing teeth a sign of aggression or fear. Do not smile at them toothily.
- They also consider eye-contact a challenge, as do most canids. Felids, however, consider eye-contact soothing (since it means you're not going to try to sneak up on them), unless they're already stalking you...in which case they usually charge, since being spotted by their quarry means it's about to run.