In the scene dated July 31, 2000, Cal is shown going to the orthodontist to have his bottom braces removed. However, in the first scene, dated June 23, 2000, you can clearly see that Cal's bottom teeth have no braces when he moves in to speak into the camera.
The goof item below may give away important plot points.
At the beginning of the library scene, the dispatcher asks the student calling "Can you see the perpetrators-the men with the guns?" But she later reveals that she did not know there was more than one perpetrator involved until both of their voices show up on the audio.