Stupidity: Marta is shown to barf whenever she lies, and Blanc knows this. He should've asked her who did it right away. The entire plot of this movie is rendered unnecessary.
Suggested correction: That would only work if Blanc knew that Marta had knowledge of who killed Harland and if he came to the conclusion that Harland actually killed himself. Blanc doesn't know that Harland killed himself to protect Marta until Marta confesses. Also, this wouldn't have solved the "who done it" anyway, since Marta had no idea that Ransom switched the vials.
Stupidity: Spoiler. The protagonist is a trained and competent nurse, paired with one of the greatest murder mystery writers. Neither finds strange in the slightest that after jabbing his vein with a dose of drugs 30 times the norm he is absolutely fine, not just conscious but even able to concoct on the spot a convoluted plot, speaking normally and quite at length, no trouble at all. He should be dead "in 10 minutes" sure, but it's not a time bomb. You'd think one would not be so blasé about slitting their own throat and the other would have to notice how amazingly unaffected and lucid the other appears to be minutes later. Not to mention that his plan would have never worked with the toxicology report, which should be routine in a suicide case also to assess the mental state of the person who left no note or anything behind.
Suggested correction: It is explained that the drug overdose will kill Harlan in 10 minutes based on the dosage. The implication is that Harlan's heart will stop, not that he will become gradually and obviously sick over those 10 minutes. Regardless, based on what they believe will happen, even if they did notice that Harlan wasn't getting sick they wouldn't have the time to test that theory. The fact that neither Marta nor Harlan thought about a potential toxicology report is a pretty major part of the plot, and it is perfectly reasonable given the circumstances. The plot was hatched on the spot within a few minutes and there are several holes in the plan that drive the story throughout the film. Although a brilliant man and a great writer, Harlan simply didn't think of everything.
She explicitly says "You'll feel symptoms in 5" and when he shuts her up putting a hand on her mouth she says "We have 6 minutes."Then his daughter interrupts them and more time is wasted. By the time when he begins his convoluted explanation of the big plan he should have already been disoriented, sweaty and the whole gamut leading to his respiratory failure. And he goes on for minutes after that. It's very true, it moves the plot along, but by what they say themselves (which is from I understand not medically accurate and contradicted also by what happens later in the movie with the second death) they should have realised that time has passed with nothing happening. You could even say it's Rian Johnson's intentional deconstruction of the artificial nature of the whoddunit contrivances! But also, just saying, one of those "Stupid actions and decisions people take in movies, which no-one would ever do in real life."
Even taking that into account, what you are saying is Harlan should have said "Hmm, a few minutes have passed and I haven't felt any symptoms, so I'm not actually poisoned. Carry on then, false alarm." It moves the plot along because Harlan isn't willing to risk Marta getting in trouble for poisoning him and they have less than 10 minutes to act. This would count as a stupidity entry if Harlan didn't care about who took the blame for killing him, but obviously he does. Remember, stupidity entries are not for poor decisions by characters, they are for minor plot holes. This being "an act no-one would ever do in real life" is kind of the entire point of the movie. Nobody believes Harlan would do this because, well nobody cares about their nurse that much. But he does.
The part I was quoting is the description of the category in the metadata on google, or if you prefer the hover text description just above this very page go by "Something just plain stupid. Not as deal-breaking as a plot hole, but something daft, like running upstairs with a killer behind them, instead of out of the front door." I call "slitting your own throat feeling totally fine after you yourself have been calling the minutes with precision earlier", pretty silly, to say the least. Again, this is all stuff the script itself unnecessarily calls attention on. If he didn't mention twice the time before, if she hadn't said that the symptoms happen after 5 but just "your heart is gonna exploded at the 10 minute mark", then, maybe, I would have simply reported the factual error that this is not how it works. It's the script itself that points out (Harlan himself says it twice) the exact minutes, and the symptoms and how they are gradual.
This still ignores the fact that they don't have time to test the theory. They would have to notice the lack of symptoms, and assume somehow that the lack of symptoms after 5 minutes must mean that Harlan isn't actually poisoned, and stop their plan right then and there. The audience knows that Harlan isn't really poisoned, but we don't find that out until later. I doubt very seriously that anyone watching this film for the first time believed, as you suggest, that Harlan obviously wasn't poisoned because he didn't show any symptoms and it was therefore stupid for him to kill himself. It seems to you to be stupid in hindsight, but I honestly don't believe, based on what the characters knew, that Harlan's action was so egregious that it constitutes a mistake in the script.
We definitely had a very different impression watching it the first time. The thought that this old man could be shot a big dose of morphine in vein and calmly think of perfect murder plans for the next minutes was 200% absurd on first view here. I could say that others thought the same but it's just anecdotical and I respect you having a different take. For the rest, it's again just the script itself drawing attention to it. From the mouth of the same character who nonchalantly slits his own throat feeling still fine. It seems egregiously stupid and contradictory.
But he wasn't shot a big dose of morphine. He got his normal meds, they only think he overdosed.
We don't know that yet. We know that, in their words, he was shot 100 mg instead of 3 (does not matter if true or not, we are fed this information and the characters believe it). Again, the whole scene would have worked if they didn't, themselves, add details. Makes the overdose sound huge, and inserting the 6 minutes mark (which means, barely 1 min till the symptoms show up) before the daughter arrives when more than half of the scene has still to be played, weakens it terribly. Some things are maybe just stupid in hindsight, like the fact that all he needed to do was to write in his own penmanship a suicide note saying he killed himself with an injection once Marta left, but the overdose bit felt absurd on first viewing.
Stupidity: Marta constantly screws up the investigation, but Blanc and the cops just let her keep helping in the mission. She also runs from the cops, causing damage to multiple cars. She messes up the footprints, and messes up the security tape. Why was she still on the investigation team after all of these?
Suggested correction: Marta is clever enough to make messing up the footprints and the tape look like simple accidents. Since she is still useful to Blanc, he keeps her on the investigation. The police believe that Ransom forced Marta to run from the police, so they don't hold her accountable for those actions.
