This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / The Last Five Years
Also, there's a line that, once the Fridge Brilliance kicks in, is almost just as heartbreaking. During the wedding song, Cathy and Jamie sing about being together "Till there's no one left / Who has ever known us apart". During the show, because of the way the two timelines work, they are only actually together onstage twice—during their wedding and at the ending of the show when their timelines finally seperate. And that reflects how they were never emotionally together in the first place…
"See I'm Smiling" and the way Cathy tries so hard to be what Jamie wants and not rock the boat. Followed by her epic breakdown when she just unloads everything that's wrong with their relationship.
Cathy: I swear to God, I'll never understand, how you can just stand there straight and tall… and see I'm crying… and not do anything at all.
Then, during her penultimate song to Jamie, "I Can Do Better Than That" (which is chronologically taking place at the very beginning of their relationship, but is at the end of the show, after the previous "you and you and nothing but you"), she sings the following:
Cathy: I want you and you and nothing but you Miles and piles of you Finally I'll have something worthwhile To think about each morning You and you and nothing but you No substitution will do Nothing but fresh, undiluted and pure Top of the line and totally mine!
She also mentions how disappointed she was that her last boyfriend left her with nothing but a "heartfelt letter". The fact that Cathy's pure, unadulterated love for Jamie has been so twisted by the end of their relationship and that she sees none of it coming is heartbreaking enough, but what's worse is that Jamie's final song, "I Could Never Rescue You", is him writing a 'heartfelt' (whether this is true is debatable) letter to inform Cathy that he is leaving her after 5 years of being together. Poor, poor Cathy. * sob*
Every single time Jamie and Cathy express how happy they are in their relationship ("Shiksa Goddess" and "A Part Of That!"), because you know from the first fucking line in the show that it's all gonna crumble.
I always found "A Part of That" perfectly fine until the last bit of the song. Something about the way Sherie Rene Scott sings, "I'm a part of that, aren't I?" seems to bring up a lot of self-doubt, which is then confirmed when we realise just how emotionally distant Jamie and Cathy are from each other.
Not a verbal one, but a musical one: Once you've heard "I Could Never Rescue You", you can never revisit the show the same way again. Hearing that wistful, bittersweet waltz melody is bad enough at the beginning of the show once you know what it portends, but then hearing it again at the end of "The Next Ten Minutes" is an absolute gut punch when you know that this is the theme of Jamie giving up on the marriage for good.
From the movie, this part of "See, I'm Smiling" can hit especially hard, especially if you've ever been cheated on or betrayed by a former significant other.
Jamie: Cathy, you're being crazy! Cathy:[visibly in tears] No, I'm not, NO, I'M NOT!