Continuity mistake: There is no knife on the table when the wife enters the kitchen at the very end, however, she walks straight to Louis, hugs him and then magically a knife appears at the edge of the table which she picks up and uses to stab him (implied - the movie actually ends at that point).
Continuity mistake: In the scene where Jonathan finds Cooper in the bathroom possessed, Cooper attacks John with a knife in his left hand. As Cooper lunges at him, John slams the door, but Cooper's left hand crashes through with little problem. But, there's no knife. The knife is now in Cooper's right hand. (01:05:42)
Continuity mistake: When the devil sets Toxie on fire, there's a group of five people standing there with empty buckets. But in the next shot the buckets are filled with water.