During the entire movie the guy Ashton plays is in love with a girl named Kaley. At the end of the movie when Kutcher is in the mental institution for "killing" Kaley, He is in the office of his doctor talking to him about his notebooks, the doctor CLEARLY says its all made up from the guilt of killing KATIE. When her name is Kaley.
This may not be a slip-up because the doctor may simply have misread the chart, or his notes, (remember doctors usualy have bad writing), also Doctors in a mental hospital, have dozens of patients, so misnaming somebody is a possibility