Sunset's Brutal Honesty, courtesy of the Elements of Harmony. This happens on such a regular occurrence, Sunset's all but given up trying to avoid it.
For example, when Human!Twilight questioned Sunset how she knew her name without even mentioning it, Sunset suddenly blurted out information about Equestria!Twilight. She clamped her hands over her mouth twice to stop what to Human!Twilight sounded like the rantings of a crazy woman.
Rarity: "What do you think, Sunset? Does this make me look good?"
Sunset: "I think it makes you look like a wh—"
Rarity: "Ooh, is that a ruby necklace?"
Chapter 15, when Twilight spills her drink on Sunset and tries to clean her up, Sunset snaps that Twilight is trying to feel her up. What follows is a magnificent combination of Motor Mouth, Suspiciously Specific Denial, and Digging Yourself Deeper. It only all stops when Sunset mercifully tells Twilight to shut up. How Sunset didn't clue in to Twilight's feelings for her after this exchange is a mystery.
Twilight: Feel you up? Oh, no, no, no! I would never feel you up! It was an accident! I wasn't even thinking about your boobs! N-not that they aren't nice! I wasn't not thinking about them in the sense that they weren't there, just the sense that I want nothing to do with them! Not that they’re bad, they’re better than mine! I mean, uh, from a scientific point of view they’re very appealing! And from a non-scientific view, I’m sure they’re nice as well! Not that I would know because I totally don’t think about girls that way. Actually, I don’t even think about guys that way! In fact, I’m asexual! Sunset: Twilight? Twilight: Yes? Sunset: Shut up. Twilight: Thank you.
The last line of the author's note of chapter 6: "Curse these humanized ponies, they drive me to write."
Sunset plays a game of chess against Twilight in the mall. When it's clear that Sunset is going to win, Twilight concedes. Sunset immediately goes into hysterics, having wanted to "crush" Twilight, with Twilight herself incredulous that Sunset is mad that she won. The resulting argument leads to the spilled drink where Twilight tries to clean off Sunset.
During the chapter where Twilight and Sunset discuss what to do for a science fair project, their discussion keeps them up to about eleven. Sunset, being Sunset, makes a snarky remark about Twilight having missed her 'bedtime'. Twilight retorts that she can go to bed whenever she wants. Cue (assumedly) Twilight's mother coming in and telling Twilight she was supposed to be in bed an hour ago.
While at Sugar Cube Corner, Pinkie asks Sunset if she knows how hard it is to catch a leprechaun. Sunset, who is magically cursed to answer any question truthfully, is so stunned that she can't answer.
After winning a small pink pony doll at the renaissance fair, Sunset names it "Twilight Sparkle" and intends to burn it. She doesn't, but instead struggles with it, as she imagines the doll vocalizing her innermost thoughts. In Chapter 25, when Sunset returns home suspecting that Twilight has a crush on her, Sunset grabs the doll and stuffs it into her dresser without a word.
A meta-example: the author posted a review of Rainbow Rocks, "co-written" by this fanfic's Sunset Shimmer. The two have fun noting the similarities between the fanfic and the movie and contrasting the fic's Sunset against the movie's Sunset, with the former referring to the latter as "Rebecca" out of indignation that "[she] thinks she can play me."
Other reviews have followed in the same style, focusing on episodes from Seasons 4 through 6 and Friendship Games. They even go back to Equestria Girls, which Sunset still has trouble watching all the way through for all the bad memories it brings up. She only agrees to do the follow-up for $100.
Artemis' mostly-asleep way of answering his phone in Chapter 35, when Sunset calls to ask if she can stay with the Lulamoons in order to avoid being sent to a homeless shelter:
Artemis: Hello, you've reached the Supreme and Mystical Artemis Lulamoon. Please leave a message after the snore.
Sunset's attempts to keep the Winter Ball running smoothly in Chapter 40 result in her dress being partially ripped and covered in dust stains. Rarity's reaction to seeing it is about what you would expect.
Rarity:Sunset Shimmer, what have you done to my dress!?
Sunset: Well, I lived a... decent life. Sort of.
Chapter 41 starts off with Twilight calling Sunset in the dead of night, waking her up. Her call is to make sure that the Winter Ball and the subsequent Relationship Upgrade that Sunset and Twilight went through wasn't All Just a Dream; Sunset softly assures her that it was all real. Twilight hangs up... then calls again a few minutes later, prompting Sunset to say that if Twilight doesn't stop calling, she'll take it back.
All of Twilight's reactions to the various aeronautic exhibits in the museum are both sweet and funny, including when she and Sunset get into a rocket launch simulator.
The chapter ends with a surprise party for Twilight, with Twilight's mom picking up on a few lines between Twilight and Sunset, asking Twilight point blank about what happened at the museum. As Twilight struggles to come up with an answer, Sunset just blurts out "Your daughter’s gay, and I’m dating her!"