Steve's accent changes between American and Irish throughout the movie. In particular his Irish accent stands out when he is on the phone to Annie or when he is yelling. However, this is not necessarily a mistake, given that the movie never specifies the character's nationality. Steve may very well be an Irishman who's been living in America for a long time. Actor Gabriel Byrne has successfully concealed his Irish accent on multiple films before this one.
When Joan writes down her phone number, she folds the piece of paper before giving it to Annie. Later, when Annie looks at this paper in her workshop, there is no crease, indicating that the paper was never folded.
In the parking lot when Annie meets Joanie for the first time, during their interaction, the front passenger-side head rest is curiously missing from her Volvo. When she is next shown driving, after leaving her first seance at Joanie's, she hears her deceased daughter's mouth click as if from the back of the car, we see the front passenger-side headrest is back in place.
In the opening graphic of Ellen's obituary, her death is noted as taking place in early April. When Annie meets Joan, Joan says a few months have passed. Then we are shown the Seance flyer with a date of April 18.
Charlie does not stick her head further out the car window than the sideview mirror, yet we hear the sound of the telephone pole colliding only with Charlie, and not with the mirror as well. In reality, driving so close to the telephone pole would result in the sound of the pole hitting the sideview mirror, and then the mirror hitting the car, before hitting Charlie. This is a very distracting mistake in what was supposed to be a key scene.
The goof item below may give away important plot points.
Near the end, when Peter goes up to the tree house, he sees the bodies of his grandmother and mother on the floor; the latter is on the right and the former is on the left. Later on, we see that the bodies have switch positions - the mother is on left and the grandmother is on the right.