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 Suspiciously Specific Denial and Digging Yourself Deeper. 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!”
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.
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, which was "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 "she thinks she can play me."