Mistaken Message

Alice writes Bob an unsigned note, typically a secret admirer type message. Carol finds the note, misinterprets it and believes it was written by David. The note continues to be passed around until all characters have misconceptions about one another.

See also Oops I Did It Again, Love Letter Lunacy.

Anime and Manga

 * A variation is played on Suzumiya Haruhi, when the class representative Ryoko Asakura leaves Kyon an anonymous note in his Inside Shoes locker. Kyon first thinks the note was written by Mikuru, but then he corrects himself: the note was a simple sheet of paper torn from a notepad, and it would be Mikuru only if the note was written in scented decorated paper sprinkled with glitter. Massive mayhem ensues. Decidedly not of the romantic variety.
 * Some of the mayhem is of the romantic variety, or at least looks a lot like it.
 * He later finds a letter actually written in decorated paper by Mikuru... only problem is, he's only partially right...
 * Another variation occurs in Love at First Sight, where a lovestruck man dictates a love letter to Yuki to Kyon. Naturally, this means Kyon writes the letter in his own handwriting. It led to some awkwardness when explaining the situation to Yuki, but it got worse when he threw it out and Haruhi saw it.

Film

 * In Charlie Chaplin's The Gold Rush, Georgia writes an apologetic love letter to her boyfriend, and he gives it to the Tramp as a cruel prank. The 1942 re-cut version changed this so that Georgia writes the note for the Tramp, making their Last-Minute Hookup less unexpected.

Literature

 * Happened in PG Wodehouse quite frequently.

Live Action TV

 * The classic example is from the sitcom Mama's Family, but most long-running sitcoms offer a variation.
 * Good Morning Miss Bliss, "Love Letters"
 * Are You Being Served, "Dear Sexy Knickers"
 * In Kenan and Kel this leads the later of the titular duo accidentally stealing someone else's girlfriend. Hilarity ensues.
 * The Comic Strip Presents film Mr Jolly Lives Next Door features an escort agency accidentally receiving a letter meant for a hitman. The message? "Take out Nicholas Parsons".
 * In Full House, a prank love letter was left to one of the girls. She distresses over it and misplaces it. Repeat for every other character on the show.
 * In an episode of My Name Is Earl, Earl helps a man who'd written a love letter Earl had prevented from being delivered by spending time with the intended recipient so she won't get involved with anyone else before the sender can get back into shape again. When she finds the letter, she thinks it's from Earl. Unfortunately,.

Video Games

 * Video game example: Final Fantasy IX, in which Eiko leaves a note for Zidane. It never reaches him, and is eventually intercepted by both Steiner and Beatrix, who each think it's from the other.
 * Incidentally, the game was greatly inspired by Shakespeare, who absolutely loved this kind of plots.

Web Comics

 * In Avalon Ceilidh meets half the cast thanks to a misdirected love letter from Ryan to Phoebe,.