(at around 47 mins) 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 Joan 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 Joan'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.
(at around 45 mins) 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.
(at around 1h 16 mins) When Annie begins the seance with Peter and Steve, the candle on the table isn't lit but once the camera cuts away and comes back to the table, the candle is now lit with the flame clearly visible.
The playhouse has lapboard siding and cedar shakes for a roof coating, yet from beginning to end, the interior shows light shining through the cracks in the vertical wall board and ceiling. This is simply not possible.
(at around 50 mins) In a close-up of Annie working on her dioramas, the lace strip on the hairline of her wig is clearly visible.
(at around 1h 17 mins) When Annie does the seance with Peter and Steve, a metal ring can be seen inside the glass on the table when it is moving, most likely used together with a magnet under the table. It is not visible in the shots before or after the glass moving.
(at around 1h 8 mins) For portions of Joan's instructions to Annie, as Annie is leaving Joan's apartment, the lips don't match the words, notably the comment that the husband and brother need to be 'in the house.'
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.