When Kenny enters the upstairs bathroom, he removes his backpack and takes a look in the mirror. His reflection shows him not only without his backpack, but in that split second he somehow loses his jacket as well.
When Amanda comes down the stairs her hair is done. In the next shot when she reaches the bottom of the stairs coming in to the yard, her hair is very flat. Then Preston says "Amanda, I love you" she turns around and her hair is done again.
In the scene where Kenny and Denise are talking in the street, every time the camera is on Denise her bangs are on her face, but every time the camera is on Kenny, her bangs are off her face and pushed back.
At the beginning of the movie, several characters (including Preston) are introduced with yearbook entries. Toward the end of the movie, we see Preston's entry in the actual yearbook. The two almost match, but not quite; the actual yearbook includes French Club, but this is omitted from the introductory bit.
When Preston looses hope after seeing Amanda being kissed by her cousin he tosses the letter he wrote to Amanda in the garbage, you see a silver Honda Civic next to the trash cans. The Brake lights and third light are flashing, it was done in a rhythmic pattern to make it look like the hazard lights are on. In reality the only way to make this happen would be to have someone continuously pump the brakes. The hazard lights on that make and model would be the lights below the brake lights and would flash orange in the same rhythmic pattern.