Continuity mistake: When Harrison Ford is watching his wife laying in the bathtub ready to drown, the camera shoots her head several times between cutaways to Ford and the running water. Watch her head relative to the top of the tub in the various shots. In some her head is much higher than in others yet her body never moves.

Continuity mistake: When Claire first sees the ghost, Norman rushes in and Claire grabs and hugs him. Her hand was just in the tub yet as she's hugging Norman, her hand is dry. She would have left some kind of a wet mark on Norman's shirt.
Factual error: When Claire pulls herself out of the bathtub, she falls right over the center of the tub. The bloody fingerprints would have been wiped away or completely smeared by her nightgown and bathrobe during the fall.
Continuity mistake: When Claire is in the tub after her husband gives her a drug to immobilise her, watch her hair in various shots. It keeps changing position even though she is unable to move it out of the way with her hand.
Continuity mistake: Claire takes a basket of flowers over to the neighbor's house. Notice when she first goes to the side door the flowers take up most of the basket and are arranged a certain way. When she turns and is spooked by the car in the driveway, the flowers are arranged differently and do not take up as much of the basket.
Continuity mistake: In one scene when Michelle Pfeiffer is in the garage looking at the photo album, we see her looking at a picture of her daughter who is in a black vest. The camera angle then changes to one looking at her from across the room and we see her turn the page. We then see her half turn a page and the camera angle changes to over her shoulder to where we see her turn the rest of the page over, but if you look closely at the page before you can see it is the picture of her daughter in the black vest again.
Other mistake: When Claire Spencer loses her husband's mistress' braid in the lake, she tells him this and Dr. Norman Spencer acts like it's the first time he's heard her talk about it as he picks it out of the water. She has, however, mentioned the braid a short time before this to him, right after the rough sex they've had.
Revealing mistake: In an early shot shown from the neighbor's house, it is established that the Spencers' house is quite a ways uphill. Some of the later shots where Claire is spying on her neighbors and looking at their home, the angle of view does not appear to be correct. The neighbors' house looks like it's a lot closer and more level with the Spencers' home.
Continuity mistake: When Claire is in the tub after Norman gives her a drug to immobilise her, she stops the hot water valve with her toe, although Norman opened the cold water valve.
Suggested correction: He explained that the water was too cold (he laid in cold water after faking his electrocution) and thus turned on the hot water. But when she turned the water off with the shower hose, the tap turned anti clockwise, so actually opened the tap more.
Continuity mistake: When Michelle Pfeiffer is walking up to her door towards the middle-end of the movie, Michelle puts the key in the door to unlock the door, but she never unlocks it. Then the lady next door comes up and scares Michelle and when they go in, the door was never unlocked but somehow Michelle still got in.
Continuity mistake: When she is paralysed and drowning in the bath she tries to use her feet to remove the plug. Watch her big toenail closely, it varies in length, indicating they probably used a foot double.
Continuity mistake: When Claire goes to the Feur's house and sees Mary's shoe on the porch, she goes to pick it up. When she does, it is in a different position than when she originally saw it.
Continuity mistake: When the bathtub is shown full, it is filled perfectly to the rim, which means the tub is level. When Claire is being drowned in the tub by Norman, the water is clearly deeper at one end of the tub, indicating that the tub is slanted.
Character mistake: Claire's behavior makes no sense and is intirely implausible and unrealistic. For example: Whenever Claire talks about the ghost, she never mentions any of her compelling observations. When she tells Norman about the seance, she incorrectly says that nothing happened. Claire wakes Norman up to look at the Feur driveway after the apparent body-bag is already in the closed trunk. What did she want him to observe? A car? An empty driveway?
Suggested correction: People already wonder if Claire is unstable right now. Her daughter left for college. She sometimes takes anxiety medication (she told the therapist). Her husband is a practical, realistic scientist. Those could all be reasons why she thinks that her observations will be dismissed. Also, reasons to not say much about a seance. Regarding the car, Norman didn't move fast enough see everything before Mr. Feur left. Again, he might simply think Claire is imaginative and nosy at this time.
Continuity mistake: When Claire gets the box and necklace out of the lake, she sees the necklace and lays it down on the open box. She goes to get the phone and then Norman quickly appears. She then later has the necklace on in the tub.
Continuity mistake: When Claire is in the bath, paralysed from the drug Norman gave her, and the water is rushing in, her head changes position: At first, her head is below the waterline, and then it is above the waterline - then Norman slips and hits his head - and then her mouth, nose and eyes are below the waterline.
Other mistake: When Claire turns the hot water valve with the chain, it's turned in the wrong direction. It was turned to open it more.
Continuity mistake: When Claire hears her neighbour crying, she calls out to her, and the startled woman dashes off to the left, so Claire (of course) dashes off to the right, but, when we next see the neighbour, she is at the right.
Revealing mistake: After Claire and Norman drop their daughter off at school, that night, Norman is in bed with his laptop, typing, but not once do his fingers touch the space bar.
Continuity mistake: At the end, Norman and Claire are in the truck, which smashed into the river. Norman has less blood on his face than in the next shot, where he has a lot.