In Atonement, the moment of catastrophe is brought about because of Briony's immaturity, fear, and confusion. At thirteen, Briony is on the cusp of womanhood, becoming slowly aware of her own insignificance in the world at large as she struggles to juxtapose her own desire for relevance with her desire for love. When she reads Robbie's crudely sensual letter to her twenty-three year old sister, Cecilia, all hell breaks loose, so to speak.

Briony imagines sinister intentions on Robbie's part.

She had read the note standing shamelessly in the center of the entrance hall, immediately sensing the danger contained by such crudity. Something irreducibly human, or male, threatened the order of their household, and Briony knew that unless she helped her sister, they would all suffer.

Briony's suppositions are framed by her immature conception of sexual attraction and her inexperience with the realities of sexual tension. Also, the author mentions that Briony is the kind of character who likes to have everything just so. Consequently, her preoccupation with order and abiding consistency is a way for her to navigate the often confusing maze of adult emotions she is not yet ready to process.

So, when she comes across Cecilia and Robbie entangled in the throes of foreplay in Chapter 10, Briony thinks of Robbie as 'huge and wild' in contrast to the supposedly delicate and frail figure of her sister. Cecilia, meanwhile, shows no signs of gratitude for Briony's inopportune entrance; if anything, she views her sister's intrusion as an annoyance at best or an act of sabotage at worst. In this, Ian McEwen expertly highlights the latent competitiveness between sisters in his portrayal of the tension between Cecilia and Briony. Catastrophe is foreshadowed when Briony does catch Cecilia and Robbie in the throes of passionate sex in the library.

Briony's feelings of betrayal are palpable; the girl who has spent her private moments trying to regain her emotional equilibrium after her cousin, Lola, usurps her role in the play, must now witness her older sister's initiation into a part of the adult world she has no experience or conception of. In her mind, her older sister has achieved some sort of significance in a transcending experience she has had no part in. At that moment, the tragedy or catastrophe of unrealized dreams, unfulfilled longings, and thwarted desires for relevance and significance in an unforgiving adult world fuels Briony's subsequent misinterpretation of what she sees.

Unable to effectively process what she has just witnessed, Briony's misinterpretation of the sensual scene is at once pathetic as well as devastating:

Propelled from the depths of her ignorance, silly imagining, and girlish rectitude, she had come to call a halt.

It is devastating in the sense that Briony's misinterpretation of what she sees is about to set off a range of events which will bring sorrow and grief into her life and the lives of others.