When Danny is kidnapped, it is overcast. Moments later, the sky is clear.
Rick favors the wrong hand when opening the door during the fire.
After Rick sprays paint thinner around Frank's workshop, he ends up setting his right hand/arm on fire. When he leaves he opens the door with his right hand and holds up his left arm (as if that was the arm in pain). Once he exits, his right arm again is the one in pain.
When Ray is having breakfast with Frank, he has a piece of toast in his hand. A second later, in the next shot, it's gone.
At approximately 1:11, Rick has thrown fluid on the walls, then looks to his right at the body of Frank. The next shot shows Frank lying on the floor, but behind him you can see a man's feet (Rick's?) walking.
Ray Coleman and Patty were watching a college basketball game on October 18 in the motel, a month before the college basketball season starts.
When police get involved in Danny's claim that Rick murdered Ray. Rick would have been easily caught because there would have been much more blood in his car, and there would have been lots of bones left over after cremation that would be easily identifiable as human. Especially so shortly after the crime with minimal clean up.
The Southport Police cars have "SG" license plates, which is for state government cars. Local police cars have "LG" plates, for local government.
When Frank and Rick are fighting in the garage, Rick knocks Frank to the floor with a paint can. The light shines onto Franks face, when Rick picks Frank up off of the floor, revealing an obvious stunt double.
In Susan's bedroom before the custody hearing there is a photograph on the dresser and the nightstand, its the same photo.
Vince Vaughn is seen smiling and laughing (out of character) at nothing when Steve Buscemi gets into the car at The Shady Tree Motel. There was no dialogue and nothing funny happening prior to this. The car then drives away still with no dialogue.
Frank visits Patty and as the camera moves over to them, with Patty sitting on the couch and Frank opposite her on a chair, the film crew is heard moving around. The viewer can notice things like footsteps and/or pants rustling as they are walking on wooden floors. This is all obvious because both actors are not making any noise and sounds besides speaking, so the film crew noise comes out in the spaces between dialogue.
When Frank uses his cell phone to call their house near the end to warn Susan and Danny to get out, Rick checks the Caller ID, which comes up Frank Morrison, but with a "410" area code, which is a Maryland number. The movie is set in the area of Southport, North Carolina, which would give Frank a "910" area code. Coincidentally, at the time of filming (prior to number portability) all cell phone numbers in Maryland used the "443" area code.
Patty only remembers Ray from being reminded of his full name, not his first name. Her line of work does not require her to know anybody's first and last name. Doesn't make sense she doesn't remember him at all from just his first name.