At the birthday party, where Sarah has killed the girls, you can see one of them breathing.
Just before Donna found her 3 daughters slain in 2004, Donna is using an advanced version of IPhone. Early version of IPhone weren't available before 2007.
When Ben is driving in the middle of the night near the end of the movie, a Canada Post postal box can be clearly seen on the side of the road, but the story is supposed to be in USA.
When Zoe answers her cell phone in the woods you can see in the left hand corner of the phone, Rogers, which is Rogers Wireless, a Canadian wireless telephone company headquartered in Toronto, not in USA.