Continuity mistake: During one scene in Camryn's Halloween birthday party (where she is wearing a silver mask), it is actually Tia playing this role instead of Tamera.
Revealing mistake: Before Paxton is hit with the 3 prong instrument, you can see the bloody holes already on his shirt. (01:01:20)
Factual error: When they go into the water, they put mouthpieces in, but, a few times, they talk to each other, which is impossible with mouthpieces in.
Revealing mistake: When the team descends to the bottom of a hole and one female member is first wounded and then dies. After checking her pulse, the others pronounce her dead and begin moving out. As the last one is leaving, the "dead" actress lifts her head up off the ground and looks left. (01:13:10)
Visible crew/equipment: There is a shot of Holly crawling through a tight space with the camera facing her moving backwards as she crawls forwards (about 30 minutes into the film). In this shot, the wheel marks of the camera can be seen on the ground in front of her. (00:30:05)
Character mistake: When Lisa is taken to the hospital, she tells Kathy that she saw the ghost of Jodie DeFeo. The name of both of the DeFeo's daughters wasn't Jodie. Their names were Dawn and Allison. Jodie was the name of a demonic pig that the Lutz's daughter Missy had befriended.
Suggested correction: While this is true in the original novel, the writers chose to change it to the DeFeo's daughter's name in the movie. Creative license doesn't count as a mistake.
Continuity mistake: When Amanda and Daniel entered the bathroom from 'Saw', Adam's body is shown with his right leg shackled. However, in 'Saw' and in Adam's death flashback in 'Saw III', his left leg is shackled. (01:14:10)
Continuity mistake: When John is driving to work and he hears Anna on the radio, he swerves to avoid a car. From inside the car you can see that there are no cars in the rear window, yet when the shot changes to an outside shot, there are 3 cars. (00:33:35)
Continuity mistake: When Constantine manages to get the mirror through the window he falls on his back. To the right on the screen a chair lies on the floor beside a dresser. A few shots later when it cuts to Constantine getting up we can see the chair has changed position as its back now covers the opening of the dresser. (00:08:15)
Visible crew/equipment: When the woman is performing an autopsy on one of the dead "creatures" the lights supposedly flicker and turn off. There is a hand visible flicking the switches on the lamp to the right side of the screen. (00:45:40)
Continuity mistake: As Rachel is preparing the peanut butter and jelly sandwiches, she places the knife on top of the open PB jar in a closeup, but in the very next shot the knife has disappeared, then reappears again. (01:23:15)
Continuity mistake: The male lead is seen reading an article in the newspaper, the headline of which says "serial killer arrested at Pendleton University," but when he reads the headline aloud, he mentions the professor that killed students at Alpine University, which doesn't match the headline from the paper he's reading.
Continuity mistake: Nick's shirt he gave to Carly is way too big on her when she first puts it on. In the next scene it fits like it's her own shirt.
Continuity mistake: When the hot blonde gets out of the water she is supposed to be naked, as 5 minutes before we saw her take off her bikini bottoms, but as she walks out you can see she still has her bottom half on. (00:20:55)
Plot hole: In the end, when Charlie is advancing on Emily while turning his light on and off, the final time he turns it on, Kathrine is in front of Emily. How could she get there so fast without being heard? There's at least 4-5 inches of water that would make sneaking very hard.
Visible crew/equipment: When Wendy goes into the bathroom, you can see the cameraman's shadow moving. (00:48:45)
Continuity mistake: When the Seagrass is attacked, the two girls are thrown through the cabin's rear windows, landing on the outside deck. When the boat's owner finds it, a camera circles the Seagrass from a distance and the rear windows are intact. When he and Elizabeth board the boat, the windows are again broken. (00:26:40 - 00:42:15)
Continuity mistake: Right after the beginning credits, Jacques the chef walks through the door holding a plate of food with both hands. When the shot changes, he is walking through the door but is now holding the plate of food with one hand and has a fork in the other and has just taken a bite.
Continuity mistake: When Chelsea returns back to the house, she runs through the front doorway and the door behind her is still open. In the next shot, the door is closed.
Visible crew/equipment: When Betsy is having the sheet pulled up over her, the lightning flashes and lights up the visibility. You can see a wire which is being used to guide the sheet.
Suggested correction: Sorry to burst your bubble here but there is scuba equipment which allows for talking whilst using a regulator.
They're not wearing such equipment. In the movie they're just wearing regular mouthpieces.
In the DVD commentary the writers explain that the divers should have been wearing full face masks which allow for communication, but the director changed it for mouthpieces, but kept the talking in. They pointed out this mistake.