After she had written the script, Sofia Coppola was heartbroken to discover that another company was already producing an adaptation of the book themselves. However, they were not happy with their script, so she showed them hers and they ended up using it instead.
At the beginning of the film, workers apply a "Notice for Removal" tag to a diseased tree. The tag reads "City of Gross Point Parks Department." This is a misspelling of Grosse Pointe, MI - a bordering suburb of Detroit - and the city in which both the book and movie take place. Jeffery Eugenides, the author of the novel, was born in Detroit and attended University Liggett School in Grosse Pointe.
When the girls are on lock-down, you see the days going by with a view of the house. This was after the tree in the front of house was supposedly cut down - yet the tree is visible in the whole season's montage.
The trivia item below may give away important plot points.
The pre-homecoming photo of the girls foreshadows their deaths. Lux is holding her hand out to feel a leak coming from above (her hand's position resembles holding a cigarette) - later when she dies, her hand is seen hanging out of the running car in the closed garage, holding a cigarette. Therese's eyes are closed, and she died from an over dosage of sleeping pills. Bonnie's arm appears to be scratching her upper back, but also resembles a hanging - which is how she died. And after the photo was taken, Mary coughs, and she died from sticking her head in the oven.