Michael and Holly - The Office
When the office is uncomfortable with Michael and Holly's PDA, Michael makes an announcement claiming that his co-workers are jealous of seeing two people in love. Holly and Michael are both caught off guard by his statement, but she returns the declaration even though they had only been dating a week. When you know, you know.
Chandler and Monica - Friends
The most romantic time to confess your love is always when your girlfriend is dancing with a turkey on her head. Chandler may have tried to take the declaration back in panic, but once it's been spoken, it's out there for good.
Amy and Karma - Faking It
Karma believed that Amy was confused and didn't actually have feelings for her, but Amy just had to make Karma understand. Frustrated and heartbroken, Amy let an "I love you," slip out while she was in tears.
Clarke and Lexa - The 100
Lexa almost declared her love, but she was able to catch herself right before dropping the final syllable. Even if she didn't finish the thought, both Clarke and the audience knew how she really felt.
Damon and Elena - The Vampire Diaries
This one may not have slipped out by accident, but it came out by accident. Damon only told Elena he loved her because he was going to erase her memories directly after. But once Elena became a vampire, her memories returned.