In the 16th minute Kathy shows Aurora pictures of some of the serial killer's victims and refers to one individual as 'victim zero.' The first victim would be correctly referred to as victim one.
In the 23rd minute when they enter their vehicle and start the engine, the driver's wide front turn signal comes on and stays lit solidly until she turns on the headlights. They should not happen.
In the 63rd minute when the boy and his mother are eating dinner while Christmas music plays, she makes the sign of the cross before eating. From seeing the perspective of someone watching the person make the sign, they would touch their head, stomach, then their left shoulder and right shoulder. The mother *drags* her finger from left should er to right.
In the 75th minute the Christmas tree which was set afire by a thrown candle tips. It would not tip so soon, let alone that a thrown candle would likely go out before reaching the tree.
In the 52nd minute the shooter shoots all four family members in the first car. When the heroes in the second car reverse and turn to escape him, the shooter is then shooting an automatic weapon. While it is not impossible that he changed the weapon's firing format to fully automatic, it seems unlikely that he would not have used the automatic setting from the beginning of his ambush.