Any situation in which a character carries out an action which is misinterpreted as a wedding proposal by another character. Typically, the latter character accepts, much to the former character's horror. Hilarity Ensues.
It can be done in a number of ways: Bob wishes to propose to Jennifer and shows the engagement ring to Alice, who assumes he's actually proposing to her; Joe wishes to propose Alice and Bob finds the ring and shows it to Alice; Bob says something which, taken out of context, could sound very much like a wedding proposal etc.
Usually played for comedy, but can be played for drama.
Common in sitcoms and soap operas. Sister Trope to Accidental Marriage (indeed, this trope often leads to that one). Can easily lead to a Fawlty Towers Plot. Compare/contrast the Mistaken for Index.
Urusei Yatsura. During a competition to determine the fate of the Earth, Ataru's girlfriend promises to marry him if he wins. After defeating his female alien competitor Lum, he says how glad he is that he can get married. Lum thinks he's talking about marrying her, falls in love with him and accepts his "proposal".
In an Archie Comic, in one of the Xmas issues, Moose shows Jughead a jewellery store window with a birthstone ring. He then asks Jughead to find out from his girlfriend, Midge, if she'd like that ring for Christmas. So later, Jug casually asks Midge if she would like the ring in the store window. Midge kisses Jughead and runs off in ultimate excitement. Turns out the jeweller had since placed a diamond engagement ring in the birthstone's spot. Since Moose was normally so jealous that he was known to hospitalize other guys for merely talking to Midge, Jughead was now seriously in deep doo-doo.
In Tim Burton's Corpse Bride, Johnny Depp's character is practicing his wedding vows in the forest because he just can't get them right. When he manages to do it correctly, he slips the ring onto what he assumes is a twig sticking up from the ground. Instead, it's Helena Bonham-Carter's character's hand, she is the titular character and believes it to be a proposal. A Back Story tells us that she was murdered before her wedding. This leads directly to an Accidental Marriage.
Dexter and Rita become engaged when Rita sees Dexter holding a ring (... actually belonging to Dexter's sister's boyfriend) and assumes it's a proposal. In a twist, Dexter goes along with it.
In the seventh Ciaphas Cain novel, Cain does this to the daughter of a noble (who's after a husband so she can stake her claim as the governer of her homeworld) by basically nodding his head and going "uh-huh?" through one conversation too many when she's hinting at it. He gets out of it when he's assumed dead and she marries the leader of a nearby planet.
When Garrett, P.I. and his girlfriend Tinnie encounter a powerful sorceress, he's worried the woman's flirty mannerisms will provoke Tinnie into a faux pas that will get them both blasted. He introduces Tinnie as his fiancee purely to defuse the situation, only to have her hold him to it and demand a ring a few pages later.
In A Princess of Mars, John Carter becomes engaged to Dejah Thoris after she calls him "my chieftain" and he responds by calling her "my princess", with him not knowing what the exchange of those terms means. It takes him some time to work out why she becomes so upset with him.
Robert A. Heinlein's The Number of the Beast. While dancing together, Zebadiah Carter and Deety Burroughs enjoy a passionate tango together. Zebadiah says "Whew! After a tango like that the couple ought to get married." Deety takes him completely seriously (not realizing he was just joking) and accepts his proposal.
In Louisa May Alcott's Jo's Boys, Tommy accidently proposes to Dora while still thinking himself in love with Nan, who had resolved to become a doctor and so never marry. He is unable to contradict her on the spot, and the effect of the engagement is to shake him out of Oblivious to Love. (Nan makes up a basket of medical supplies for a wedding present, knowing Dora will need it for his accidents.)
In P. G. Wodehouse's Jeeves and Wooster stories, Bertie once tried to act as a go-between for Madeline and Gussie. It convinced Madeline that he was speaking for himself. Not only did she consider them engaged, later, after that engagement was broken and she was engaged to another man, she broke it off and declared she would instead marry Bertie and make him happy.
In The Vicar of Dibley, the vicar (a woman) is asked by her love interest "will you be the one to marry me?", by which he means to join him and his partner in Holy Matrimony, rather than to become his wife. She misinterprets this and, when he introduces the other woman as his fiancee, yells whilst laughing "I don't think so!" then realises and apologises, pretending it was a joke.
Also in How I Met Your Mother, a sparkling wine flute containing a ring was delivered to Ted and Robin's table while they were on the first anniversary date, rather than the table next to theirs. When Robin sees it she yells "Noo, no no no no no no no, No! No, no no no, no, no, No!"
And it's Played With when Barney "proposes" to Abby, who was in on the plan, to show Ted how annoying he can be with girls. Abby isn't the sharpest nail in the box and thinks it's a real proposal, telling her dad "It finally happened, daddy!", Barney believing her to be 'doing a bit'.
Friends: Joey finds Ross's engagement ring, which Ross was going to present to Rachel and ask her to marry him. Joey turns around while happening to be on one knee, still holding the ring. Rachel says yes to Joey.
In a sketch on Monty Python's Flying Circus a man goes to a male marriage registrar and asks the registrar to marry him to his fiance, but the registrar misunderstands and thinks the man is asking him to marry him. He accepts, although he does have a wife already, but he amiably decides to get a divorce. Another man comes in and asks the registrar to marry him and the registrar says he'd already accepted the other man's proposal. In the end it turns out they're all Straight Gay (including the first man's fiance and the registrar's "wife"); they all get married and live together happily ever after.
In the Doctor Who story "The Aztecs", the Doctor accidentally gets engaged to Cameca by making her an offer that in her culture is a euphemistic marriage proposal.
Mr. Humphries and Mrs. Slocombe on Are You Being Served?: he was going to suggest something else to her, but she thought he was about to propose marriage, and when he started with, "I propose," she interrupted with, "And I accept!"
Done by James in Big Time Rush. Apparently giving a rose to a princess is a marriage proposal in some foreign countries.
In a sidequest in The Wind Waker, Maggie thinks Moe the moblin's letter to her is a marriage proposal. In reality, it only says "I want to eat you for dinner."
Inverted in Ocarina of Time: When Ruto gave the Zora Sapphire to Link, in the eye of a zoran, his acceptance of it would mean that Link accepts her proposal. However, Link only needs the sapphire to open the inner chamber of the Temple of Time. So Ruto proposed to Link, but he interpreted it in a different way.
There's an Urban Legend surrounding a Coca-Cola ad. In the ad a man proposes to his gal by having a skywriter write "Marry Me Sue" in the air. According to the UL a woman named Sue in the area where the filming took place had a boyfriend who said that he wasn't going to propose to her; but if he did, he'd do it in a spectacular fashion such as hiring a skywriter.
Five hats means that five tropers think it is ready to publish.
You are saying that you think this draft is ready to be published. That means the description is not ambiguous,
it doesn't duplicate an existing trope, there are at least three examples, and the title makes sense.
Is that what you meant to do?
You are saying this draft has a ready-to-publish hat it does not deserve and you are taking it back.