Mistaken Declaration of Love

So, Bob has finally worked up the courage to tell Alice how he feels. Oh! There she is, and he finally manages to spit it out... Only to find out that's not Alice, that's Carol!

Whoops!

This can have one of two main variations:
 * Carol also has feelings for Bob and is overjoyed, Bob now has to find some way to convince her it was all a mistake.
 * Alice just saw what happened and is likely to not be pleased.

In the worst case, BOTH of these things happen. Once in a blue moon, it dawns on Bob that Carol really is much better for him than Alice.

Usually played for comedy, but occasionally for drama. Compare: Love Letter Lunacy (which is mainly a specific variant caused by problems with the postal service), Accidental Marriage

Anime and Manga

 * School Rumble: This is how Harima and Eri start off their relationship, as Harima mistakenly confesses to Eri instead of Tenma. Although Eri hadn't had any feelings for him before that, his heartfelt confession—in contrast to all the shallow confessions she got from the other boys and easily brushed off—freezes her into embarrassed panic. Harima is likewise frozen in panic when he realizes what happened.
 * Potemayo also has an example, when one of Those Two Guys "practices" confessing to one of the Moe Blobs only to do so right in front of another girl...
 * How Ataru of Urusei Yatsura ended up with Lum. Also a case of Accidental Marriage.
 * And a similar thing happened in To Love Ru, with someone (usually Lala) getting in Rito's way every time he tries to confess to Haruna.
 * Ranma ½'s Ryoga finally worked up the nerve to tell Akane how he felt and why he'd used a love-inducing artifact -- he'd meant it for her, and hit Ranma instead. But his declaration was also accidentally given to Ranma, who didn't take it well.
 * In one arc, Ranma and Nabiki become engaged, instead of Ranma and Akane. After Ranma finds out that Nabiki's been playing him this whole time for a profit by saying she loved him, he wants revenge. So, he decides he'll make her feel weirded out by pretending to be in love with her. He invites her on a date, and eventually, Nabiki ends up being chased by Ranma in a maze. Ranma's determined to see it through, so he pulls her through a hedge and screams that he loves her. To his great distress, it turns out to be Akane, not Nabiki, in his arms. Cue major freak out. (This example is possibly inverted in that he wanted to tell the non-love interest, not the love interest, that he loved her.)

Fairy Tales

 * In Swan Lake and its ballet adaptation,

Film

 * A deleted scene of Star Trek depicted Kirk apologizing to an Orion woman about getting her in trouble, only to find that he's talking to a different Orion woman.

Literature

 * In Jill Mansell's novel Millie's Fling this occurs. Hugh decides to tell Millie he loves her, however, when he proposes to "her" she is dressed in her gorilla suit and it turns out to be her best friend - because the stripper was too busy to fill in for Millie and the history teacher wouldn't fit in the costume. It turns out Millie is in London watching her friend the recently divorced novelist have sex with her arch-enemy in a toilet cubicle. It Makes Sense in Context. However, the best friend is uninterested in Hugh and is evilly amused - and then proceeds to tell everyone she knows.
 * PG Wodehouse's Bertie Wooster made a declaration of love to Madeleine on behalf of his friend Gussie. She took it as coming from him and forever after would decide to marry Bertie and make him happy whenever her love life went awry.

Live Action TV

 * An episode from a later season of Coupling reveals that  until the very first episode of the season.
 * On Gossip Girl Serena gives her mask to Jenny at the Masquerade, causing Nate to confess his love for SERENA to Jenny. Blair, his girlfriend at the time, dumps him when she finds out, and Serena didn't recognize his true feelings until much later.
 * In Friends, during Ross's wedding to Emily, he mistakenly says "I take Rachel" as his bride instead of Emily's name. She wasn't very happy.

Video Games

 * In Wario Ware Smooth Moves, a football player works up the courage to admit his love to Mona (who passes by him... twice) only to explode on a stray cheerleader.

Web Comics

 * Lampshaded and averted in Its Walky. Danny meets with his ex, Sal, with whom there is still a mutual attraction. Danny, however, has moved on and is now in a stable relationship with Billie. He informs Sal that he intends to propose to Billie soon, and Sal congratulates him and asks to see the ring. Danny refuses, saying he knows how this goes; "I pull out the ring, and Billie comes walking around the corner..."

Western Animation
"Exile: "Please consider therapy.""
 * Road Rovers: The Rover's plane is about to crash, and Blitz grabs the hand next to him and declares his love for Colleen. The hand was Exile's. His response?


 * From The Swan Princess, Derek gave one to who he thought was Odette, but was actually Rothbart's henchwoman in disguise. Made more painfully that the real Odette, in swan-form, was just outside the window and trying to get his attention from making the mistake.