"Amanda's" hair is back to its same length (and style) when Walter visits "her" in the psychiatric hospital as it was before "she" had the barber cut it. While "she" does say "she" has been there for five months, and apparently five or six months have transpired from the time of the trial, there is no way the hair would have grown from above the ears to down below the shoulders in six months - hair grows about 1/2 inch per month, so it would be about 3 inches longer than before. The hair should have only been slightly longer to show that amount of time has passed.
At the beginning of the movie Steve takes three roses from the lobby of his apartment building. The roses were open when he picked them out, but were closed buds when he handed them out to the women.
In the last scene, the CPR given by the doctor is just above the left of the navel. This is the incorrect place to do it.
When "Amanda" first met Mac she baffled the confused doorman in the elevator. The door was somehow closing. However Mac did not touch any button. Amanda's right hand was around the calling buttons but these are unable to close the door.
When Margo fires her revolver at Steve, there are no rounds loaded in the gun.
When Amanda punches Walter, the sound comes before the hit.
How did the fact that Amanda doesn't seem to exist in any legal way come up when she was being hired to replace her "brother", when the police were pursuing her, and when she went to court, and when she went into the mental hospital? She has no social security number, no work history, no record of her birth, no medical records. On paper, she doesn't exist, but this never comes up.