Second Act Breakup

Alice and Bob are the two subjects of a rom-com. They've met in a manner most cute, had a few laughs, done an obligatory "Falling in Love" Montage and look all set to book the church/synagogue/registry office/volcano/whatever.

Then, about 60 minutes into the 90-minute movie, there's a sudden wacky misunderstanding where one of them is Mistaken for Cheating. Or Alice realizes her enemies will come after Bob to get to her, and she has to Break His Heart to Save Him. Or maybe they just have a colossal argument and both storm off in a huff.

Cue about 20 minutes of moping and trying to move on before they both realise that all they want to do is wait for the other to get out of the bathroom (or maybe join them in there) for the rest of their lives. If the argument was caused by one person, they may need to grovel before they can be forgiven. Then cue the tearful reunion.

Contrast with Plot-Mandated Friendship Failure. Compare with Third-Act Misunderstanding.

Examples of Second Act Breakup include:

Pretty much any Romantic Comedy ever made. EVERY. FREAKING. ONE.
Extremely common in sequels where the lead couple had their Relationship Upgrade at the end of the first movie. Bridget Jones: The Edge of Reason is a good example of one where it happens even though the breakup is completely pointless and has no effect on the plot.
Sailor Moon features a second-season break-up when Mamoru has prophetic dreams that compel him to leave Usagi for her own safety. They only get back together in the last few episodes.
In I Love You, Man the bromance and main storyline feature this. The main character separates from his fiancee for a day or so, but the main Second Act Breakup is between the main character and his new friend.
Fushigi Yugi pulls out one of these every ten episodes. At least. In a 52-episode series.
The book of About A Boy subverts this by having Rachel react with amused tolerance when Will reveals that Marcus is not really his son. The movie plays this trope straight while using most of the same lines spoken in a different tone.
Edward leaves Bella in the second book of the Twilight Saga for her safety and she zombies out for ten pages that are literally blank.
The Night Huntress books have two; midway through the first book, Bones leaves Cat until she can figure out how she feels about vampires. At the end of the first, leading into the second, Cat writes Bones a Dear John letter.
Happens in the prime minister David and household staff member Natalie in Love Actually. He saw the American president kissing her on the cheek, thinking Natalie was never interested in him. It wasn't until he saw the Christmas card from her, saying that she was his that he realized that she returned his feelings. Cue him trying to find her house to apologize and her happily accepting it and them making up by kissing at the back of the school's stage. Only for the curtains to rise.
Happens in the Avatar: The Last Airbender episode "The Beach." At the end of the second act (the party), Zuko pisses Mai off so much that she breaks up with him. They make up at the end of the third act (the campfire), and refreshingly, there is no groveling.
The Fenris romance in Dragon Age 2 has one literally in Act 2, right after Fenris and Hawke consummate the relationship, when he freaks and walks out on Hawke. Depending on the player's actions toward Fenris after said incident, it can actually be permanent.
The X-Files: I Want to Believe has a weirdly low-key example. Scully and Mulder break up, sort of, but they both seem rather unsurprised to find that they can't really stay away from each other for long, and at the end not only is there no groveling, they don't really even apologize and make up—they just quietly go back to acting like it never happened. One gets the impression that they've done this before and while they were both upset, neither of them really thought they were going to stay broken up.
Happens in La Bohème because Rodolfo knows he cannot care for his girlfriend, the fatally ill Mimi. He accuses her of cheating on him or wanting to cheat on him, justifying this idea by saying he caught her looking at other men, so they will break up and she will not feel obligated to live with him in squalor and the cold, which is exacerbating her condition.
Happens in Back to The Future Part III between Doc Brown and old west schoolteacher Clara Clayton.
In The Valley of Horses, Ayla and Jondolar undergo a temporary break-up when Jondolar finds out about Ayla's past and shows his prejudice against the Clan. In The Mammoth Hunters, Ayla and Jondolar have problems for most of the book which come to a head in the middle when Ayla agrees to meet someone else. If it weren't for the fact that Ayla and Jondolar go through most of The Plains of Passage until they have their fight, making it more of a Third-Act Misunderstanding, the series would have used the exact same plot point three times in a row.
The July 4th Debacle in Holiday Inn, which results in Jim and Linda breaking their engagement, followed by Jim closing the Inn and Linda heading to Hollywood with Ted. It takes Mamie lecturing Jim before he heads to Hollywood to win Linda back.