These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Fanon Discontinuity: Since Carla's relapse and death is shoehorned into a tiny paragraph on the last page of the book, it's easy to pretend it just never happened in order to retain her character development, since the epilogue comes off strongly as a Very Special Episode reminder that Drugs Are Bad — as if the reader couldn't have figured it out already. Moreover, the last sentence of the book (in which the author reminds the viewer how many people die from using drugs) pretty much hammers in the message rather than wrapping up Carla's story in a satisfying way.
Harsher in Hindsight: The entire book, once you know what the epilogue is: Carla's innocence and dreams of a happy future; her love for her parents and siblings; her desperate attempts to get off of drugs.
It Was His Sled: Carla dies. Understandable due to this book being used as both scare 'em straight material and high school curriculum for years.
Seinfeld Is Unfunny: Comes off strongly as moralistic, preachy anti-drug material nowadays that mostly invokes a seen-it-all-before response from most young people who read it. But when it debuted in the 70s, it was so shocking that it was censored almost immediately.
Tear Jerker: Both of Carla's reunion with her parents qualify, as well as the deaths of her grandparents. The ending, though abrupt, can also bother you for a long time after reading it.