In olden times, ordinary people would be very careful what they said because words have power. You didn't have to be a priest, oracle or wizard to cast spells with words, anyone, anyone could potentially make something happen just by saying so.
The reasons are both supernatural and mundane. Lending weight to the proverb "you're only as good as your word", you have an obligation to follow through on your promises, lest you face social (or even supernatural) consequences. This is why there is nothing more dangerous than making promises lightly (even impossible ones). Firstly, some fool hero will complete the task and since your word is binding you'll be compelled to keep your promise. The other is because you are never alone... someone or something will be listening, and may well decide to give you exactly what you wish for. More than one such tales starts because someone said something they shouldn't have, or started making very ill advised boasts.
Speaking in the heat of the moment is doubly dangerous, it's not uncommon for Curses to be cast this way to everyone's sorrow.
At the same time and on the other end of the spectrum, taking a vow of silence and undertaking some task or penance can break a curse.
Super Trope to:
- Advice Backfire
- Be Careful What You Wish For
- Black Speech
- I Know Your True Name
- Joke and Receive
- Literal Genie
- The Promise
- Rhetorical Request Blunder
- The Scottish Trope
- Speak of the Devil
- Tempting Fate
- Threat Backfire
- Words Can Break My Bones
- Word Power
Examples not otherwise included in a subtrope:
- In a spin-off book of the W.I.T.C.H. comics, Taranee's brother, who's been spending lots of time with his girlfriend instead of her, ends up showing up much too late to see a basket game with her favorite team like he promised. She refuses to buy his (heavily implied to be true) excuses about car trouble and snaps that she wishes he'd just disappear. She immediately regrets those words because, as a Guardian of the Veil, her words have a lot of power.
- In Jesper Who Herded the Hares, Jesper's brothers lied about what they were carrying to the old woman. They thought. When they got to the castle, they found they were carrying what they had said.
- In Whuppity Stoorie, the woman says she will give the green gentlewoman anything she wants in return for saving her pig. What she wants, it turns out, is the baby.
- Exact Words in Johnny Rivers' "Secret Agent Man." The following line? "You'll give yourself away."
- The Witcher 3 features no shortage of curses applied to hapless folk, sometimes entirely by accident. In-universe, strong feelings and words are enough for any idiot to put some sort of curse on another idiot. Though it usually requires something like seriously betrayal leading to death or the like for a curse to happen all spur-of-the-moment. Even in-universe, no one is entirely sure of how such a thing happens, but Witchers like Geralt are proficient at breaking them.
- At some point in Noob Valentin tries to pick up a girl in MMORPG by telling her "I bet you're a bomb in real life". The girl in question is a Mad Bomber in-game, and not interested at all.