When Matt and Carl confront the couple in their car, Hope is wearing a reddish color blouse. When the scene cuts away to her being dragged into the forest, moments later, she is wearing a light blue blouse.
Matthew says that they shaved his left leg in case they couldn't find a vein. In subsequent shots, it has clearly not been shaved.
When Sister Helen is being pulled over for speeding, the camera shows the view through the rear view mirror. In this shot, the cop car behind her is driving on a straight road with a dashed center line. The next shot show's both cars on a curved portion of the road with a solid/dashed (passing) center line.
In the flashback scenes of the murder, the dead couple have their hands bound behind their backs as the killers flee. In the flashbacks of the couple's bodies, the next day, both are seen with their hands unbound and their arms at their sides.
In some scenes where Helen is talking to Matthew through the grate, her face is pressed up against the wires but in reverse angle shots it is not.
The chemicals used in lethal injections in Louisiana are administered manually, not by a machine as in the movie.
While Louisiana's lethal injection gurney is capable of being put in an upright position, inmates are only strapped to it when it is down.
Matthew apparently spent only 6 years on death row. Most prisoners actually serve more than 10 years on death row due to the appeals and complected paperwork as well as securing enough evidence to have proof if the inmate was innocent or not. Even though Matthew admitted he raped and killed the young girl as well as killing her boyfriend it is not severe enough to be executed in a few years. Timothy McVeigh however was one of the very few to be executed 6 years after the Okaholma bombings which killed more than a hundred victims as it was one of the largest mass murders in US history.
The real killer (Willie) was executed by the electric chair, not lethal injection.
During the last meal Matthew is told his execution will go ahead as scheduled by the warden. In reality inmates can still get a very slim chance of a stray seconds before execution as the Supreme Court can halt the execution once the inmate is strapped down.
As a nun, Sister Helen has her ring on her right hand instead of her left, as it should be for a married woman.
At the end of Hilton Barber's conversation with Sister Helen (Susan Sarandon) outside the courthouse just after the first clemency hearing, he says what has been misheard as "Excuse me, Susan." The line is actually "Excuse me, Sister."
When Hope Percy is about to be shot in the head you can clearly see Walter Delacroix still breathing, even though he had already been shot in the head by Poncelet.
When Sister Helen and Sister Colleen are discussing Matthew's funeral, Sister Colleen, thinking about his funeral suit, asks Sister Helen how big Matthew is, as though she's never seen him in the flesh before. But Sister Colleen was at the Pardon Board hearing, sitting right behind Matthew. She would have known his size.
When Sister Helen and Mr. Delacroix are first speaking in his living room, a crew member wanders into the background outside the window and then quickly runs out.