In one of the final scenes, Vivan hits Madeline in the forehead with a hammer. Madeline is seen on the floor with a cut over her right eye, with a fair amount of blood. Vivan then leans towards her to see if she is alright, but there is no more blood on her forehead. When Madeline bites Vivian, the blood returns.
The goof item below may give away important plot points.
After Madeline kills the doctor, she drains copious amounts of blood from his arm into a bottle for Grace. Even if the blood were still warm in his body, it would not flow as freely and as much as is shown. Without a heartbeat, blood flow stops. Also, the doctor has already lost a lot of blood through his head wound before Madeline makes an incision on his arm and he's lying on a cold bathroom tile floor, making the amount of blood Madeline gets from him even more unrealistic.