(at around 33 mins) When Hutch is looking up information on Loomis' death on the computer, you clearly see "www.dogpile.com" in the address bar. Few shots later when the camera tracks out to show you the screen, there is nothing in the address bar.
(at around 18 mins) When everyone first boots up the game and Swink puts on his gaming glove, he puts it on his right hand. Yet, when the group is customizing their characters, the camera switches to a close-up of his controller, and the glove is now on his left hand. In the following shot, the glove is back on his right hand.
(at around 1h 7 mins) When Hutch shoves Abigail into the crypt and closes the door, there is no artificial light source. After Hutch fends off the phantom carriage and enters the crypt, there is still no artificial light source. Upon pushing the lid of the sarcophagus open, there is still no artificial light source. When the camera pans back out to them pushing the lid, a lantern appears out of nowhere and is lit.
The tattoo on October's arm always stay on the same arm but in every scene that we see it, the tattoo changes position on arm. Near the start of the movie the tattoo is in the center of her arm on the underside, a few shots later the tattoo has moved a few inches off to the side.
At the beginning of the movie, the mansion is shown with Loomis playing. Later when the rest of the group plays, the layout of the mansion is completely different. Towards the end of the movie, the actual mansion Hutch and Abigail are in and the mansion Swink plays in the game are different from the others. The layout of the game mansion and the live mansion never match the previous.
(at around 31 mins) When Fin is playing the game on the couch, Swink, Abigail, Hutch and October go stand to his right. Swink is standing in front of Abigail slightly to the right, and in the next shot he's right behind Fin (unless you watch the Director's Cut which shows Swink moving to behind Fin as he smokes a bong).
The goof items below may give away important plot points.
(at around 39 mins) When Fin is dragged by the horse and carriage roughly twenty feet, you can clearly see him trampled by the horses, run over and dragged by the wheels of the carriage. Upon stopping, his clothes are completely clean and intact, and only small amounts of blood are visible on his head and neck. When October finds him and cradles him, his clothes are still intact, but there are more wounds and carriage marks across his body.
(at around 51 mins) [director's cut only] When the detective is killed in his truck, his head explodes, covering the entire windshield with blood. However, in the very next shot, which is a side-view shot of his truck sitting in the parking space, you can clearly see all the way through the now totally clean windshield to the other side.