Continuity mistake: During the infamous leg un-crossing scene when Sharon Stone is being interrogated, a cigarette in her hand disappears and then reappears.
Plot hole: This is another movie where the cops are too dumb to go out by themselves. The woman who icepicks the first victim (we see) to death would have left enough forensic evidence on the scene to convict her ten times over - skin, hair, sweat, saliva, vaginal fluid, possibly blood, and they all contain DNA in abundance. She had vigorous, sweaty sex with this man and she didn't clean up afterwards (and she couldn't have done so thoroughly enough anyway) so she's left calling cards all over the place. The killer also handled the icepick (which the police take away in an evidence bag) with bare hands - her prints would be all over it. She might as well have left a signed confession, but they can't even identify her. Sharon Stone, for instance, leaves her fingerprints (on the chair and fixtures in the police station) and her saliva (containing epithelial skin cells which are an excellent source of DNA) on the cigarette butt she discards, also in the police station. She went there of her own accord and these artifacts are legally accessible by the police. It is obvious to anyone that the women who had sex with the victim killed him, and Catherine is most certainly a suspect. They don't have enough to charge her but they would if they did a simple series of tests on the dead man's body - and if she didn't do it, that would eliminate her as a suspect. They don't even check.
Continuity mistake: During the big sex scene you see red marks on Michael Douglas's back. These red marks are from when Sharon Stone scratches him. The thing is, she scratches him a minute AFTER you see the red marks on him for the first time.
Continuity mistake: During the interrogation scene, Stone has her legs crossed right over the left then suddenly left over the right and a second later (during the shot where she wears no underwear) right to left again.
Visible crew/equipment: Michael Douglas spots Sharon Stone's car after the high-speed highway chase and as he walks up to her you can see the reflection in his sunglasses of the cameraman walking in front of him. (00:42:10)
Continuity mistake: In the scene where the actual murder takes place the blonde woman stabs him in the neck. The body has no blood on it. The second stab the body is covered with blood. This all happens in one eyeblink.
Continuity mistake: As Michael Douglas and his partner drive Sharon Stone to the police station for questioning in the shots of her she's sitting on the left of the rear window. When Gus Moran looks at her in the rear view mirror she's on the other side next to a large police light. Throughout the scene she moves left and right between shots on the back seat. (00:23:00)
Visible crew/equipment: Jeanne Tripplehorn along with Dr. Myron and Dr. McElwaine meet with Michael Douglas back at the station after Roxy's death and equipment shadows can be seen moving all over the walls. (01:28:00)
Continuity mistake: Douglas opens a mailbox to check the mail for clues. He then puts the mail back and closes the box, a second later you hear him closing the mailbox again.
Factual error: Michael Douglas is lying on the couch watching the Jeffersons. Jeanne Tripplehorn arrives and they talk. In the background the theme song to the Jeffersons is playing, but it is the song from the beginning of the show rather than the closing theme.
Continuity mistake: When Roxy drives Sharon Stone's car off the road it lands on a yellow hose at the bottom of a construction site. When Michael Douglas runs down into the construction site the car is nowhere near the yellow hose. (01:26:10)
Continuity mistake: Before Sharon Stone leaves Michael Douglas' apartment the box on the left against the far wall near his front door is empty. Several shots later when Gus Moran brings him a pizza, the box is full of stuff. (01:05:00)
Continuity mistake: Sharon Stone finely comes out of Hazel Dobkins' house and her car is dry. There's no indication that Michael Douglas hasn't been sitting there waiting for her but his car is soaking wet. In the next shot it's dry also. (00:42:55)
Continuity mistake: Michael Douglas goes back to Sharon Stone's house where she invites him upstairs as he watches her break up ice cubes with an ice pick. If you look closely at the painting above her fireplace, you can watch the different times of day go by between shots in the reflection of sunlight off of the pool and onto the picture. (00:47:10)
Continuity mistake: When Roxy runs off the bridge you see the Lotus crashing over with its low beams on - when you see the car fly off the bridge its brights are on.
Visible crew/equipment: As Roxy follows Michael Douglas after he leaves Mac's Diner, the shadow of the cameraman can be seen moving on the license plate of Sharon Stone's car. (01:23:45)
Continuity mistake: Someone moved the pencil holder on Jeanne Tripplehorn's desk between shots. It's there in the beginning of the scene then it just disappears. (00:14:10)
Continuity mistake: Sharon Stone leaves the top of Michael Douglas' back all bloodied with her sharp fingernails. After the lovemaking scene, Roxy talks to him in the bathroom and we see the reflection of his back in the mirror. The scratches on his back are now much lower and not anywhere near the right place. (01:13:25 - 01:16:10)
Continuity mistake: In their love making scene Sharon Stone leaves Michael Douglas' back all dripping and bloodied with her sharp fingernails. In the next shot she rolls him over onto a pillow but when he gets up there's no blood on the pillowcase. (01:13:20)
Continuity mistake: As they discuss the murder of Jonny Boz Jeanne Tripplehorn introduces Dr. Lamott to the investigators and the book that is right in front of him moves sideways on the desk between shots. (00:17:45)
Answer: Because she's insane? And very ill. And dangerous. A pure bred psychopath. She got off screwing with him, his head. She has a degree in effing with you. Beth was just a bonus. Unlucky. Collateral damage. Unfortunate. Catherine was, is a clinical psychopath. She had no reason. Or it was fun for her. She was one twisted broad. An evil dame. (IMHO).