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.
Plot hole: When Connick has Hudson hanging in the toilets at the start of the movie he grabs the first cop who tries to help her and says he cannot decide whether he should shoot or stab the cop, choosing eventually to do both. At the end of the movie, the copycat does this as well but he makes a mistake: after grabbing Hunter he tells Hudson about not being able to decide if he should shoot or stab but the copycat already had a dead cop's body placed in the toilet in the exact the same way Connick dropped the cop. This makes Hunter the second cop. For somebody with such an eye for detail, the copycat should have done the 'can't decide' monologue with the first cop in front of Hudson, but he is already dead on the floor when she wakes up.
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.
Revealing mistake: The fight scene between Peter Kurten and Andy outside the gay bar is viewed from behind a metal gate. Peter smashes Andy's head into the gate twice. The bars of the metal gate bend towards the camera as if they are made of rubber.
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.
Revealing mistake: Harry Connick, Jr. obviously has black ink or something on his tooth to make it look like his tooth is broken. You can tell while he's talking that what you should be able to see behind his teeth if there really was a gap is not there.
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.
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.
Factual error: When Helen is launching the video that shows the girl turning into worms (the killer's next victim later found at the dumpster), typing "tomorrow.avi" as she does would have returned an error in Linux. She should have typed "call tomorrow.avi" to launch the video. (00:43:00 - 00:44:00)