Factual error: Almost at the end of the film, Poncelet is marched to the death chamber, with Prejean's hand on his shoulder. They are led past an open door - which leads to the viewing room, where the witnesses will congregate. Some of them are already in the room. This is nonsense - it is inviting a bereaved relative to attack the condemned man. In fact the corridor to the death chamber goes nowhere near the witnesses' area, and Prejean would have had to have been escorted back through two guarded doors to reach it.
Continuity mistake: When Sister Helen is being pulled over for speeding, in the view through the rear view mirror, the cop car behind her is driving on the left side of the road, with the police officer driving in the right front seat, on a straight road with a solid center line. In the next shot, both cars are on the right side of the road, with the police officer sitting in the left front seat, on a curvy road with a dashed center line.
Trivia: The movie is loosely based on Sister Prejean's experiences with two real death row inmates, Robert Lee Willie and Elmo Patrick Sonnier. Both men were electrocuted. In the movie, however, Sean Penn's fictional character was killed by lethal injection--because the image of a man strapped down to be injected allows for all that Christ symbolism.
Trivia: When Sister Prejean is back at the convent after Matthew's appeal is denied and his death sentence has been determined to go forward, she is playing games with some kids. A playing card is tossed her way, which she grabs and exclaims "Look! I got an ace!" In fact, the card she's holding is the ace of spades, commonly referred to as "the death card" - a not so subtle pun.