Continuity mistake: As Mike scrolls through the microfilm, we see his view as the pages scroll from right to left. The shot then reverses to show his face through the scrolling film, but while the film is now properly reversed, it's still moving right to left in our view. But this reversed angle should show it moving left to right.
Continuity mistake: When Mike is speaking with Olin the first time, Olin has a handkerchief in his chest pocket, and two 'points' stick out. But after Olin tells Mike that he thought the "father was a bastard", there are three points sticking out for the rest of the conversation, even through the elevator ride.
Continuity mistake: In the scene Where Mike Eslin's hotel room has frozen over and he is laying on the floor talking into his recorder he notices his computer turns on and his wife is talking to him. As Mike tries to get up you see his head touch the alcohol bottle, in the next shot that is further away from Mike you see the bottle is much further away from his head then it should be.
Continuity mistake: When Mike goes to the window to call for help, his doppelganger across the street is mimicking Mike's every move. After waving their arms in mirror-image synchronicity, Mike leans to his right slowly, and so does the doppelganger.but it should have leaned to it's own left to mirror Mike's movement. All its other movements are exact mirror images.
Continuity mistake: When Mike steps off the elevator, he is between the elevator doors and a dark statue in the middle of the hallway. The statue is seen behind him, slightly to our right. When the elevator doors close, Mike's location is different as he begins to walk down the hall. Relevant to the earlier shot, he is much further to our right and farther from the elevator doors, as now has to walk around and past the statue he was originally already past.
Continuity mistake: As Mike catches the attention of his mimic in the apartment window across the street, he makes two distinct gestures with his hands to signal distress: the first being the "hand phone" imitation and the next pointing downwards. Right before the mimic begins reflecting Mike's movements, Mike is shown making this sequence of hand motions. However, as the camera pans to the mimic we can see its movements are off-sync with Mike's, incorrectly continuing to point downward when it should be mimicking Mike's phone gesture.
Plot hole: Time inside room 1408 moves differently than time outside it, as evidenced by the fact that Mike spends weeks or even months in what he thinks is his home in California, while in reality he is still "in" the room. The Hotel's manager states that no one has ever lasted more than an hour inside 1408. He goes on to cite many examples where the guests died after just a few minutes. No previously sane individual commits suicide after spending a few minutes in an eerie room, obviously, so the only explanation is the one offered by the "receptionist" over the phone, albeit in fewer words: guests in room 1408, while experiencing their own time frame inside the room, are in fact reliving the same hour over and over according to the rest of the world's time frame. This being the case, Mike's ex-wife should not have showed up at the climax since Mike's hour had just started over, undoing the call he placed to her earlier in the film but later in the hour. Whether or not the film makers intended this inference is irrelevant as the movie plainly lays out these conditions and must therefore adhere to them lest a plot hole be created. [This keeps being debated back and forth - ultimately there's not going to be any absolute resolution, so it's being locked down as a mistake and viewers will just have to make up their own minds - Jon.]