Continuity mistake: There's a clear mix-up in editing during the car chase. Marta checks her phone twice for Blanc's calls as she tries to get away from the police. The first time the phone signals 9:36 AM, the second time 9:34 AM. (01:31:50)

Knives Out (2019)
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Directed by: Rian Johnson
Starring: Toni Collette, Chris Evans, Daniel Craig, Ana de Armas
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Benoit Blanc: I suspect foul play. I have eliminated no suspects.
Trivia: Michael Shannon has played Jaeden Martell's father twice on screen, in this movie and Midnight Special.
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Answer: She didn't drain either of the vials so unless he actually measured how much of each liquid there was to start with, he was unlikely to notice that the amounts weren't quite what would have expected. He knew enough to know that the switch could lead to a death, but he's not anal enough to note every tiny aspect of the situation to verify that nothing's gone wrong.
Garlonuss