Interrupted Declaration of Love

Alice has been in love with Bob but has been working up the courage to tell him. When she finally does, she manages to get out "I" before something cuts her off.

Unlike Can't Spit It Out or Aborted Declaration of Love, this trope is when a declaration of love is cut off by something outside the character's control. Usually the audience will only hear the "I" while the "love you" is left unsaid. It might even be left unclear as to whether or not the statement even is a declaration of love at all.

The interruption might come from a Moment Killer. In more serious instances, it might come from the character losing consciousness or dying. If two characters have had a lot of UST then this can be a good way for writers to taunt the fans.

Examples of Interrupted Declaration of Love include:

Comics

In Empowered, Mind████, gives this to Sistah Spooky, who arrives precisely "three seconds too late" to save her from burning up in a descending space station.

Live Action TV

In The Sarah Jane Adventures, Clyde is trapped on a spaceship running out of air. He records a video for his friends and, addressing Rani, says "I always...always..." before he loses consciousness.
The Doctor and Rose in the Doctor Who episode "Doomsday". She manages to spit it out, but he gets interrupted. Ouch.
On Zoey 101, this becomes somewhat of a Running Gag for Chase, who has been in love with his best friend Zoey since the day he met her. He tries valiantly several times to tell her how he feels, only to be interrupted—usually by the friends who keep telling him to reveal his feelings. Ironically, for all the times he tried to tell her on purpose, he ends up telling her completely by accident.
The Professionals. Bodie and Doyle are driving a New Old Flame of their boss George Cowley to the airport after she's survived an assassination attempt, and she begins to say, "Tell George I..." before being drowned out by a police siren. She doesn't repeat herself, and the two debate afterwards what they should tell Cowley. They decide not to mention the comment.

Manga

Ranma ½: Ranma Saotome pretty much makes it his mission to interrupt and shoot down all of Ryoga's attempts to confess to Akane. For hilarity, Ranma is usually in drag, acting like a girl, and just being a Manipulative Bastard in general.
In Cardcaptor Sakura this seems to be what is happening with between Touya and Yukito with Nakuru constantly butting into the conversation on purpose at the exact point that it seems that confession is about to take place seemingly as she fancies Touya herself. It's actually somewhat more complicated Touya is actually trying to confess that he knows Yukito isn't human... AND that he loves him and wants to sacrifice his own magic powers to save Yukito's/Yue's life, but Nakuru/Ruby Moon wants to steal Touya's power before he can do so.

Video Games

In Solatorobo, Red to Elh in one sidequest. She says she hasn't been able to feel happiness or love in years, but has finally begun to feel them again with him. He says he's got something he needs to say to her as well, but then the simulation ends and he loses his train of thought, though he remembers vividly that he was going to say something important. She decides she'll just be patient and wait until he can tell her himself. Of course, that was Merveille's plan all along.

Western Animation

Avatar: The Last Airbender: Toward the end of the second season, the Gaang splits up and Aang decides it's finally time to tell Katara how he feels. He manages to get the word "I" out before Sokka unwittingly jumps in and gives him a noogie.
My Little Pony Friendship Is Magic has this in "Secret Of My Excess", with Spike about to tell Rarity he has a crush on her... and Rarity herself interrupting him.
- It happens again in "Hearts and Hooves Day", with Cheerilee's "You have something stuck in your teeth" moment.