Revealing mistake: In the scene with the games console the controller isn't on and it is either not attached to a PS3 or the PS3 is off. You can tell because if the controller was on there would be a solid red light and even if it was just charging there would be a blinking red light.
Factual error: SPOILER: Amy Dunne returns covered with blood. After being at the hospital, and in the interview/questioning, she is still covered with blood. After being medically examined, she would have been washed clean, not just dressed over all the mess. But it gets worse; when returning to Nick in their house, she has a crisp hospital garment, but is still covered in blood! And it gets even worse; when they are in the shower, the blood on her body, which by now would have been at least a day old, would have been dried, caked and brownish color, but what she washes off is very fresh.
Plot hole: SPOILER: When Amy comes back, doctors would have checked her non-existent head wound from the "abduction", from where she supposedly lost that much blood. That would and should have revealed all of Amy's lies.
Continuity mistake: Some scenes were shot after Rosamund Pike had gained some weight, following the plot of the book correctly. However, they were edited together very badly, so that the extra pounds (most visible on her face) come and go several times during the montage showing what she did during the first two days of her "kidnapping." Pasty face with puffy cheeks, and sleek face with taut skin over jutting cheekbones switch back and forth in what amounts to a matter of hours in the film's plot.
Plot hole: Amy calls the Police about the shed before going to Desi. They watch Nick's TV statement together. The shed is only searched by the police after the statement. Which would imply the police sat on the report for several days before investigating.
Factual error: Amy uses her pregnant friend's urine to fake a positive pregnancy test. The hormone HCG which is tested for in pregnancy tests is only found in the urine during the early stages of pregnancy. The friend was quite heavily pregnant so she wouldn't have HCG in her urine by that stage.
Continuity mistake: Fairly early in the movie, Margo comes to see Nick, who is upstairs in a bedroom. The press is outside clamoring, and Margo locks the deadbolt on the front door. After her talk with Nick, she's in a hurry, rushes downstairs and opens the door without unlocking the deadbolt, although they've added the sound of a deadbolt unlocking.
Factual error: Amy leaves a lot of her blood in the kitchen. Now, I've donated blood quite a few times and I can assure you that after that much bleeding nobody does all the things Amy did right afterwards. It wears you out completely and overcoming it requires resting as well as eating and drinking properly. Huge loss of liquid and big drop in blood-sugar level.
Continuity mistake: When Amy kills Desi by slaughtering him, the amount of blood on her left shoulder disappears and reappears, depending on the shot.