When Jeff starts writing the note to Diane that he's going to look for her, he puts no dash after "Di". When he puts the note on the windshield, the handwriting is different, with a dash after Di.
In the tunnel, Jeff closes the door when Diane is looking for the flashlight. In the next shot, the door is open, without anybody opening it.
When Jeff and Barney are done fighting, the position and the amount of blood on Barney's face vary from shot to shot.
When Barney and Jeff are fighting on the stairs, a metal chair on a landing is knocked down several steps. As Jeff continues to kick Barney, the chair appears back on the landing, in its original position next to a door.
During the opening, the handlebars on the bicycles on top of the vehicle change direction several times.
When Rita is trying to guess the code to open Jeff's novel, the characters she types are shown. Computers never show the decryption passwords on the screen. They are always blocked out with null characters (Asterisk, circles, "X"s, etc.) to prevent some standing over the users shoulder and getting the password.
When they ran out of gas, there were bikes on the top of the SUV. Instead of walking back to the gas station he should have ridden one of the bikes.
When Jeff asks Nancy at the counter for coffee she suggests Milk, he refuses but she winds up bringing him Milk anyway.
Nancy is trying to convince Jeff to get some sleep before traveling on the road again. She refuses to serve him the coffee he wants. The last thing she says before pouring the milk is "One milk, coming up."
When Jeff looks up to see Diane's grave it has a fresh mound of dirt above it. she has been dead for three years. The dirt would be flat .
EDIT: "Barney" shows up at Jeff's house with the keys to the Jeff's car that Diane had when she disappeared. He may have had to dig up the grave to get the "proof" for Jeff..
The Macintosh computer has no cord attached.
When Jeff and Barney first meet and Jeff throws Barney down the stairs, the stunt double's face is blurred out.
When Barney meets his daughter at the bus/train station, the camera angle widens the bus pulls up, and extras are visible at the top of the stairs, waiting for their cue to walk down. As the scene progresses, the woman in jeans and a red sweater and her male companion are coming down the stairs as Barney and his daughter begin to walk up the stairs.
When Barney goes to Jeff's apartment, the reflection of his face in the outside car mirror as he gets out of the car would only be possible if the mirror was aimed at the sky, making it useless when driving.
(at around 19 mins) Barney is shown wearing a cast when he first sees and follows Diane into the gas station. However, in the flashback later, Barney has already discarded his cast when he first encounters Diane.
Behind the large black truck at the gas station, a crewmember is lying in the street, waving at traffic to pass.
Barney is in his car, parked downhill on the north side of Pine St., between Pike Place and 1st Ave. at the Pike Place Public Market. He sees an attractive woman walking up the hill toward him, exits the car, and asks her where the post office is. She says it's "off of "Hanley St.," and tells him to go up the hill to 1st. Ave., two more blocks east to 3rd Ave., turn right to the south, go two blocks south and at the intersection, on the south east corner is the post office. No Seattle post office is off Hanley Street.
Most gas stations are equipped with close circuit security cameras.
Jeff nor the police make any reference to checking them for suspects after Diane's disappearance.
Jeff is obsessed with Diane's disappearance, devoting much time and money in an effort to find out what happened to her. However, there is no indication that any of Diane's family members or friends ever took an interest in her disappearance or provided Jeff with any assistance in trying to locate her.