Continuity mistake: When we first see Anna's mom, the bell is on her left hand. But in the scene where Anna is down in the beach house at night and sees her dead mother, the bell changes to her right hand.
Continuity mistake: While Anna and Rachel are inside the restaurant, Anna's straw changes positions numerous times without her even touching the drink or the straw. (00:32:40)
Continuity mistake: During the scene when Rachel is driving Anna to town in Rachel's BMW, the overhead shot of the open convertible shows the car has a manual transmission. Later, when Anna is fleeing the house in Anna's BMW, the car is an automatic, as seen both by the different shifter, and the fact that Anna has to shift into Park to restart the car when it stalled.
Continuity mistake: When Anna meets Matt for the first time after her return she ties his boat to the dock. Then when Rachel interrupts them, Matt drives away in his boat, without untying it. (00:25:20)
Continuity mistake: Matt meets Anna at the pier on his boat. His left hand is holding on to the rail in the first shot and on the glass in the next shot from his side. (00:24:15)
Continuity mistake: When Alex is calling about Rachel's background checks, she has turquoise nail polish on. When it shows her holding Rachel's driver's license, she has no nail polish on her thumbnail. Although this could be correct and an Easter egg because Alex was already deceased, it could be Anna's thumb holding the license. (00:46:00)
Answer: The sedation started wearing off enough for Anna to grab the knife to kill Rachel.