When Noah is at the supermarket buying cigarettes, he pulls out of his pocket a bunch of junk such as coins, trash, and an old pack of cigarettes and dumps it on the counter. In the next shot of the counter, all the junk is gone except for a coin, and the old pack of cigarettes.
When Anna is coming up behind Samara just before putting the trash bag over her head, we see both hands at the same time and there is no trash bag in sight
When Noah is walking towards to the TV when Samara is coming out of the well, he is barefoot. When he falls backward into the shelves after Samara comes out of the TV, he is wearing flip flops.
When Aidan is drawing in the car with Noah, his clipboard changes color from blue to green.
At the funeral, when Aidan is in Katie's room, the closet door opens and shuts repeatedly between shots
When Rachel is holding the tape at sunset just before watching it, the shadows and red light move across her and the room downward; however shadows cast by a setting sun would move upward not downward.
The counter numbers are messed up on both the original tape and the copy. This is not possible with a VHS tape - no such time code is recorded on the control track like Noah, who is a video expert, says. Additionally, Noah says that the control track can be used to tell what made the recording, also not true. The control track actually just tells the VCR how fast the tape should play and when to advance one second. It is also used to determine the correct tracking of the video heads. There should be nothing wrong with this track - otherwise the tape would play at the wrong speed and the video would vanish into static. Or the VCR would keep searching for a healthy control track if the tape was just loaded.
Despite the fact that Mr. Morgan's suicide by electricity would have overloaded the circuit breaker protection for the entire house, shutting down the power, there is still electricity in the home and the outlying buildings immediately after his death.
There is no such thing as a "tracking window" in videotape terminology, as the A/V lab clerk says. The term "tracking" is used to describe the adjustment of the relationship between control track pulses and the start of a video track (video tracks are written diagonally by a rotating drum, which Noah and Rachel mess with in the previous scene). It's a phase adjustment, the correct phase chosen by the VCR is basically an angle, and all VCRs, even the most basic home units, can read the tape with a control-track-start-of-video-track phase of -180 to 180 degrees (in other words, the full range). Having a wider read head would actually introduce noise and make the video tracks less readable. The clerk/technician makes it sound like videotapes store film frames, and higher-end machines can read larger images. Furthermore, the overscan area of a VHS tape (or any analog composite signal for that matter) is nowhere near large enough to hold a whole other image.
Katie's mother tells Rachael that she spent four hours searching online and couldn't find one single case of a 16 year old's heart just stopping. Sudden Cardiac Arrest affects approximately 2,000 people under 20 every year in the U.S. Even with 2002 internet search capabilities, it would have been very easy to find many cases of a kid's "heart just stopping".
As Rachel is driving to try to warn Noah, she has her cell phone in her right hand. In the next shot, she has both hands on the wheel. She actually drops the phone to take better control of the wheel to avoid traffic.
When Rachel is searching the Internet for information, the address in the browser points to a file stored on the desktop of a Windows 98 machine. C:\WIN98\Desktop\....etc...
In the group photo of Anna Morgan with the old women, the shadows on her face reveal that her image has been artificially added to the picture.
When Rachel picks up Katie's photos a monorail is seen above heading in the south direction, moments later another monorail passes going in the same direction on exactly the same track. This would not be possible as the first car would've had to return to the stop on the other end in Seattle Center, unload and reload passengers, before being able to make another trip south.
In the penultimate scene, Naomi Watts' natural Australian accent can be heard on the line "What did I do that he didn't?"
Rachel has a very basic conversation with her sister at the wake about Katie's death, as if she'd barely been around her sister since the daughter Katie died. Yet the two seem close enough that Rachel would likely have been with her sister as much as possible. It's common for screen writers to create awkward scenes where characters transmit information that the viewers need to hear, but that don't quite capture how the characters would actually have exchanged the information. In this case, Rachel finds out everything she needs to know from her sister and Katie's friends in one tight scene.
When Samara's father attacks Rachel and hits her in the head, there is no blood. If he hit her that hard, she would've bled.
On the ferry, when the people gather at the stern to see the horse that fell overboard, a little girl next to Rachel screams, but the sound of the scream continues even after the girl stops and breaths in.
In one scene, there is a shot of a small clock ticking, but the hand is moving in a smooth motion. The make and model of the small clock does not tick, it is perfectly silent.
Towards end of movie, Rachel returns to find Aiden sleeping on the floor. She picks him up and when she turns around, by the front door, now in front of her, a shadow outline of a man is seen, a crew member.
When Noah breaks into the mental institution's record room and is looking through Anna's files, there is an arm with a blue sleeve to his right, even though he is alone.
After watching the Samara medical video in the Morgan home, Rachel chases Richard Morgan to the stairs and the lighthouse illuminates a crew member's shadow on the wall. Richard is visible at the top of the stairs.
When Rachel is looking through the newspapers for what time all of the teens died, she sees a time of 10:00 p.m. in one of the papers. According to Associated Press style, it should have read 10 p.m.
At the psychiatric hospital, Noah tries to pass himself off as Richard Morgan. At this point in the story, Noah should know that the real Richard Morgan would be at least twice his own age, and no one would ever believe he was who he said he was.