When Preston carries Jessie out to his truck in the rain (with her wrapped in the green army blanket), the distance the driver's door is open varies between shots.
About ten minutes in, Jessabelle is lying in a four poster bed with curtains and she thinks she something through the curtain and pulls it back, but in the next camera shot it is shown as closed.
When her father goes to burn the tapes he pours on gasoline. gasoline can soften and ruin some plastics. When the tapes are recovered from the trash it would be surprising they were playable.
When Jessie and Preston investigate what is reflecting across the lake, the boat they drive is registered in North Carolina, where the film was shot, instead of Louisiana, where the film is set. (at around 53 mins)
In Louisiana, there are two Feliciana parishes: East Feliciana and West Feliciana. In the movie, you see a sign saying "Enter Feliciana Parish." It doesn't say which one.
When Jessabelle's father picks her up and takes her to his home, the father pushes a large hutch out from in front of the room's door - suggesting that he shut the room up when her mother (Dad's wife) died early in her life. The bed coverings would be decades dirty as well as everything in the room.
Just before the title, the semi-trailer truck is speeding far too fast to make the 90 degree turn that is only 150 feet down the road that it is heading. That semi would have crashed making that turn anyway.