Other mistake: During the revelation of the second death, when the killer opens the toxicology report they just retrieved, the date is revealed; it says it was issued on 09/27/2018. But the events of the movie happen in November (it's Sunday 18th Nov when Marta wakes up with all the reporters at her door). It should also be noted that when a close-up of the report was shown earlier (when Ransom was looking at the anonymous letter brought by Marta) that copy had a big stamp in a corner saying 11-17-18, that in this copy is totally absent. (01:56:55)
Other mistake: Marta sinks her white shoes in the mud to cover her tracks. We see that the shoes are, as they should be, all dirty to the sole and welt. They are clean again in the following scene though (on a sidenote, the shoes have been cleaned after the murder then, and she or her mom missed the bloodstain - it's very much possible, it just adds a little implausibility). (00:57:20)
Answer: To add slightly to the other answer, evidently some of the blood in the scene had to be digitally removed for the film to secure a PG-13 rating, which explains why we don't see any actual spray/gush. But we are to assume that a drop managed to splash onto her shoe when he slit his throat.
TedStixon