Corrected entry: In three separate scenes in the movie Marta is said to be from Uruguay, Paraguay, and Brazil.
Corrected entry: The second scene shows Marta bringing Harlan his coffee first to his bedroom, and when he is not there she goes upstairs to his den only to find him on the sofa with his throat slashed. This does not make sense since she later tells Blanc the true story and how she saw Harlan slash his own throat. Therefore the second scene where she looks for him and then surprised to find his body makes no sense. Furthermore it is unlikely that any blood would have gotten on her shoe since she was a distance away in the doorway when Harlan slashed his throat. Unlikely that the blood would have splattered that far away.
Corrected entry: Ransom switches the medicine in the vials. His expectation is that Marta will look at the vials and inject accordingly. He does not know, nor can he expect, that Marta would know how they "feel" as opposed to reading the labels. If Marta had looked at the labels first, but "felt" they were wrong somehow, she probably wouldn't have given Harlan the medicine. It isn't until after she injects the medicine that she "sees" her mistake. There is no way for Ransom to have expected this outcome.
Corrected entry: He switches the medication with one syringe, that is impossible. One container is emptied in the syringe and one container is full. Leaving him with one empty and one full container and a full syringe with nowhere to put it in.
Corrected entry: Being an accomplished mystery writer and assuming that he had been injected with 100 mg of morphine, Harlan should have realised that his suicide would have been suspicious once they did the toxicology analysis.
Corrected entry: When Ransom interviews Marta at the diner, he has no clue that she actually gave Harlan the correct dose, yet the final reveal suggests he knew she was innocent at this time. It's only at the very end that we learn that she gave Harlan the correct dose because she can tell the difference based on the viscosity of the two liquids and only then that Ransom would have known that she was innocent. At the time of the interview at the diner this incorrect conclusion takes the audience down a fork in the plot that is totally incorrect based on this fatal mistake.