Marissa and Ryan -- "Thank You."
So, this seems to happen a lot, but that doesn't make it any less awkward. On The O.C. Season 1 Episode 14, Marissa accidentally blurts out "I Love You" while caught up in the moment of being in Ryan's arms, everything they've been through this past year fresh on her mind. They both stiffen in their embrace, and Marissa sort of tries to backtrack, thinking she's freaked Ryan out. Her fears are not alleviated when Ryan thanks her. She seems kind of pissed, and who wouldn't be? Sure, it was polite, but Ryan is still racing against the clock to tell Marissa he loves her before the ball drops New Year's Eve. Thankfully, he makes it just in time. And she thanks him.
Ted and Robin -- "What?"
So, this one is a bit of a cheat, because what else was she supposed to say? Is there any good response to being told "I Love You" by a guy you just met on the first date? On How I Met Your Mother Season 1 Episode 1, Ted meets Robin and goes on his first date with her, which was going pretty well until he said he loves her. His kids, his friends, and Robin were all shocked and confused by how strong Ted came on. Honestly, if she said it back, we would probably judge her a little. Still, couldn't have been fun for Ted.
Mindy and Josh -- "See ya in chemistry."
On Drake and Josh Season 4 Episode 4, Mindy Crenshaw tells Josh that she loves him. He tells her he'll see her in chemistry and promptly slams the door in her face. Josh lets his brother Drake convince him that Mindy is just trying to distract him from his science project as the two have always been academic rivals. Between Josh's hurtful doubting of Mindy's motives and his inability to suss out his feelings, this lack of reciprocated "I love you" leads to an inevitable break up. While the couple eventually gets back together, this was a hard one for fans at that time.
Dan and Serena -- "Dan, I'm not pregnant, so you don't have to say that ..."
On Gossip Girl Season 1 Episode 13 a pregnancy scare leads to Dan professing his love for Serena. She shrugs it off, thinking he was just trying to be a good guy and not taking it seriously. He spends a large part of the episode trying to say it for real, and when he finally does, all she can say is "okay," which he says is "not okay." After examining the source of her trust issues and a monologue from Dan about why he loves her, Serena is finally able to say it back. Despite everything she learns about Dan by the end of the series, Serena loves Dan Humphrey, always has, always will.
Leonard and Penny -- "Thank You"
The last stop on the gratitude train, Leonard tells Penny he loves her On The Big Bang Theory Season 3 Episode 19 while the two are cuddling in bed post-coitus. Unlike Ryan, who just couldn't say it, Penny is not yet sure she feels it. And unlike Dean, Leonard doesn't dump her for not being able to say it back. He does pressure her a little, and while they seem to get past it at first, Penny ends up dumping him. She realizes that maybe it is unfair for her to string Leonard along when his feelings for her are more profound, and she can't promise she'll ever get there. The two get back together years later, and Penny finally says "I Love You" to Leonard on The Big Bang Theory Season 6 Episode 8.
April and Andy -- "Dude, shut up! That is awesomesauce!"
On Parks and Recreation Season 3 Episode 7, April tells Andy she loves him. He is totally psyched, and he tries to high-five her after telling her that it is "awesomesauce." April is, understandably, pissed, and Andy spends the rest of the episode trying to find out what he did to piss her off. Ron finally points out what should have been obvious; April is pissed because Andy didn't say "I Love You" back. So he tells her that he does and that that is what makes the sauce so awesome. This one was just really a failure to communicate and Andy being Andy. He had no problem saying it; he just didn't realize that it wasn't implicit.