Revealing mistake: When Ashley Judd slams Tommy Lee Jones' car and tips it over, she drives off and skids dramatically to a stop. Just before she skids, you can see several skid marks on the ground from practice or previous takes.
Revealing mistake: When Libby goes to the house to kill her husband with the gun she checks the bullets. You can see the firing pin is drilled out, rendering the gun inoperable.
Answer: It's obvious it was him. Nick lured Libby to that specific mausoleum, knocked her out, and dragged her into the crypt before closing the doors behind him. He then paid off the kid who acted as a decoy. Nick was motivated to get rid of her (though it was implausible that he would have left her alive before putting her into the casket). There was no-one else who could or would have done that.
raywest