When Sara is leaving to see Paige her mother Monica stands up and tells her bye holding the paper she was reading against her side after Sara tells her bye she is walking to the vehicle (from a zoomed out shot) you can see that Monica is standing on the porch with her hand on her hip and the paper is no longer in sight.
When the parents try to drown the villain in the sink, he manages to pull the stopper and nearly completely drain the sink, but when they activate the automatic disposal, the sink is once again filled to capacity.
Francis gets stitches from the doctor for the gash in his nose. There's a shot of him with the blue stitch strings poking out of the wound. But, as soon as the scene is done, the stitch strings are gone for the rest of the movie.
The man before Justin arrives is checking out at the store, Paige says that his total is $5.25. The man hands her one bill, but she does not give him any change. (The quarter could easily have been in his hand without us seeing though)
After Mari hits Krug with the rock, she can clearly be seen running away with her shoes ON. Later, after she is shot swimming away from her attackers, we are shown a close up of her foot with her shoes OFF.
The boat in the film for which they could not find the keys was a newer model Ski Nautique. These particular boats DO NOT use keys. They use a four digit start code which you must input into the key-less ignition panel. You can in fact see this panel in one of the first shots of the boat which shows the dash. It is marked by a big red START button.
When the electricity is blown out, Emma is seen stirring the pot on the stove. The stove would have also turned off, being an electric stove. While this is true, she may have been stirring the hot chocolate simply to quell a harrowing and/or awkward moment, instead of just standing there not doing anything. Also, even though the stove is no longer on, there is still residual heat and the hot chocolate may still need to be stirred.
When Mari is swimming to get away, they are firing from the edge of the lake, she is swimming away from them. His final shot hits her on the shoulder. Later after she is found by her parents, they show the bullet hole near the top of her shoulder, a wound that would have come from someone standing behind her, not at a near 90 degree angle to her back. A real shot at the angle depicted in the film at that location on her shoulder would have just entered and exited, creating a furrow not an actual hole with no exit wound.