Goofs

When Grace falls into the corpse pit, her face becomes grubby and dirty: (in particular, large dirt mark on her lower left side cheek): as she ascends the ladder to escape and emerge, her face has become much cleaner: finally after getting out, now resting outside the pit, her face smudges change from her left side shot, to her frontal face shot, as being smudged to all far cleaner again ..

While Grace is scurrying around on her knees in the kitchen hiding from the Butler, he puts a kettle on the stove and lights the gas. Less than 15 seconds later, the kettle begins to whistle to show that the water has boiled. No kettle could boil water in such a short period of time.

The fire should have spread much faster than it does.

The character in the opening scene is shot through the heart with an arrow. He should have been dead within seconds and not able to shout and gesticulate for his life as he then does.

When the family is watching the car crash on the smartphone, it shows the same slow-motion camera effect seen by the audience. This would not happen with a live feed camera broadcasting the event.

However, this was very likely played for laughs.

Each of the family members drink from the same cup; however, every one who drank react to its contents at the same exact time.

However, they all drink in a very short amount of time and different people's guts don't act the same (depending if they had an empty stomach or not, etc.). Plus, this is played for thrill and laughs.

Thirty minutes in, Grace and Daniel are hiding behind the bed from the rest of the family. There is a mirror opposite the two in the corner of the room, which would have revealed them to the family, but no reflection is present.

Obvious CGI hole in Grace's hand, a gun shot can not produce such a wound.

Grace incapacitates Stevens and then drives off in his car. While the car company rather quickly shuts off the car, she does spend a few minutes driving away, presumably at high speed (based on the engine noise). And yet after she stops, Stevens shows up almost immediately. There would be no way for him to reach her that quickly on foot.