Continuity mistake: In the last scene on the roof when Holly Hunter shoots the serial killer, he falls backwards. The gun is in his left hand and the path is next to his left hand. The camera then goes to the rear of Sigourney as she walks toward Holly. Only now the serial killer is on the other side of the path and the left hand holding the gun is no longer next to the path.
Continuity mistake: At the first crime scene, when MJ walks into the bathroom, two candles are lit, and look to be about half burnt. The forensic says the victim has been dead for 8 hours. Those candles must have been VERY tall, if they have burnt for 8 hours. And I doubt the police lit them to brighten the scene.
Continuity mistake: When the oriental kid gets the gun from desk, there is a pen laying lengthwise on the right hand side and a yellow pencil at an angle widthwise. When he shuts the drawer, both the pen and the pencil have been neatly slid against drawer front and a piece of white paper has appeared.
Continuity mistake: When Hudsen is talking to Kate as "She Doc" in the chat room when she has her first panic attack, she is typing away but if you watch closely you can see that the keys she is hitting do not correspond to the words that are on the screen.
Continuity mistake: Hunter is in Weaver's house and Weaver types some code on a black pop-up box MS-DOS style. This box has a black background and white letters, and the computer has a blue desktop screen. However, Weaver's glasses show a big white reflection. The only white on the screen is of a pop-up window, but it is semi-hidden by the MS-DOS window.