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.
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.
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.