The Polaroid camera used to scare off the monsters has a flash strip on top of it with 10 single use bulbs in it. Sally shoots off 2-3 dozen flashes on a device that is only capable of 10. The direction and location of the used, cloudy flashes changes from shot to shot as well.
During the ending scenes when the family is fighting the "faeries" its supposedly night time during a thunderstorm, in the middle of the woods but there is clearly light shining through the stain glass window, along with other windows in the house.
While stuck in traffic, a construction worker is seen holding a circular stop sign, which is used by construction crews in Australia. US Construction crews are required to use octagon shaped stop signs.
The movie often offers close-ups of the BMW's "Rhode Island" license plate, but the numbers are red, like Massachusetts license plates. For over 15 years Rhode Island licenses plates have had blue lettering with a blue background featuring a wave graphic.
When Sally smashes one of "The Fairies" between two book cases in the library, the creature's body should serve as evidence of their existence to her skeptical father, yet the body is never again referred to.
Kim ends up tangled in the ropes of the fairies and dragged down into the basement. This tragedy is a loose end. No police report is filed, no other characters are shown to react to this and the aftermath of this event is never built up on.
The goof item below may give away important plot points.
While supposedly alone, the worker is slashed, beaten and stabbed to the point of near death. Yet nobody once questions how this "accident" could have happened or how he managed to end up with scissors sticking out of his neck.