When Lindsey is with her sister Sadie in the flower shop and is pulling the petals from a sunflower, it's clear that the sunflower changes between shots.
When Ben Murphy comes back to the apartment, he enters the first door on the left, which is the door that Sadie Jones's neighbor came out of earlier in the movie. Later in the movie the first door on the left appears to be a storage closet, and Reverend Frank goes in the correct second door.
When Reverend Frank is playing the Ten Commandments quiz game with the children, he flips the "coveting" card to its answer-revealing side on the board behind him; but in the closeup of his face, it's flipped back to its answer-hiding side. In the next shot, it's back to its correct, answer-revealing position.
When Ben climbs in the window of Reverend Frank's house the window changes from one with a smaller piece of glass and a large frame to one with a larger piece of glass and the frame holding the glass is smaller. Also the size of the window seems to change to a bigger window overall when you see Ben coming through.
During the scene where the couple first meet the Pastor, the pastor's Family Feud-esque Board has 10 answers, but his controller has only one button. Therefore, he shouldn't be able to select between the different panels when they need to be flipped. It may be arguable that he could have it set to reveal in a predetermined order, but it's doubtful, considering that it seems like some answers were given in random order (Thou shalt not covet thy neighbor's wife and Thou shalt not kill).
When the choir boy is sent to bug the bedroom, he picks the the lock on the door. However, the deadbolt lock can clearly be seen as he picks a lock below it. When the door is opened, we see that there is no other lock below the deadbolt, so he was picking a doorknob. He would have needed to pick the deadbolt lock, not the doorknob, to open the door.
There are a few scenes where they arrive at the church and traffic is going two ways. This was filmed in Long Beach and in reality, 3rd Street, is a one way street west bound only. They blocked traffic and parked cars going the wrong way to make it look like a two way street.
In the scene where Ben and Sadie profess their love for each other on the beach, one of the rotations of the camera reveals a receiver for a wireless microphone.
Although the film is set in Chicago, Los Angeles street signs are visible, including one for 12th Street; Chicago's 12th Street was renamed Roosevelt Road before World War II.