The shot where the fake blood splatters the camera lens during the murder of the driver was actually an accident, but the shot impressed director Alexandre Aja so it was left in the film. The graphic shot was cut from the theatrical release though, but was restored for the DVD release.
The trivia items below may give away important plot points.
According to the director, the screenplay version of the film had the entire movie from Marie's version of events. You didn't find out she was the killer until the end of the film after Marie defeats the killer and rescues Alex, when it would cut back to the Hospital Room and you see Marie in handcuffs. A police officer would roll in a TV and show Marie the video of her killing the gas station attendant and ask if she wanted to change her story. The real version of events was meant to be another movie entirely. However, when the writers presented the script to Luc Besson, he convinced them to change the ending to include the last reel of the film revealing Marie as the killer, instead of the last five minutes.
The original concept for the film was for it to take place entirely at Alexia's house over the course of the night. Aja and Levasseur decided that the story needed more locations, but wanted to keep the idea of it taking place all in one evening.
The scene of Marie searching for Alexia in the cornfield originally ended with Marie catching a glimpse of the killer's rusty truck in the center of the field, but the shot was cut to keep the film a bit more ambiguous.
The rifle that is later used to shoot Tom can be seen hanging over the fireplace in the shot where we see Alexia's father through the study window and Marie walks by outside. It's a subtle but startling clue to the films climatic revelation.
When the killer first appears on screen (pleasuring himself in his truck) the human head he tosses out the window is Alex's. Note the dried blood under the nose. The same as when Alex is discovered chained in her bedroom