Factual error: Opening the blank envelope, Marta finds the fragment of the toxicology report. It is signed "Office of the chief medical examiner, Norfolk County, Massachusetts." But then it writes also the address of said office, which is in Marlborough. Marlborough is also the city when Marta resides, and where the lawyers' letters we see come from. But Marlborough is also a city in the Middlesex county, not Norfolk. (01:28:25)
Factual error: The damning vials of medication are in close-ups. It is printed on the label that we're talking about 20 ml vials. The morphine vial is 5 mg/ml. When Marta says that she gave Harlan 100 mg of drug she is then wrong; unless she administered the full vial. The two vials have in fact different concentrations: the Ketorolac is in a 30 mg/ml solution, so Marta would have administered 600 mg of ketorolac tromethamine, not 100. The maximum dose for a geriatric patient is 60 mg per day; even the fact that she'd administer it in IV for days for just a pulled shoulder is definitely overdoing it anyway - and she puts even morphine on top of that. (00:36:00 - 01:49:00)
Factual error: The drug in Marta's bag is incorrectly labeled as Ketorlac, when it's Ketorolac, with an extra O. It's the name of the molecule, not a brand name that could have been altered for legal reasons - and they mention the commercial name just minutes later during questioning, with no alteration. (00:36:00 - 01:49:00)
Chosen answer: She is holding cake all 3 times, actually, so the only difference is that the first time she speaks. It can be a case of unreliable narrator, but I'd write it down as a mistake (deliberate, probably, as the phrase is important for the plot and they didn't want to hammer it in), since as you said, the other times even when they tell things to the police in a different way from what happened, the details tend to stay consistent, except for parts that are obviously made up, such as who is putting down the birthday cake for Harlan Thrombey therefore appearing subservient and not as close (when Richard tells the story, it's Walt and his wife, when Walt tells the story it is Richard and his wife).
Sammo ★