When Jay and Rebecca wake up on the couch they are both barefoot. She opens the blinds and then sits at the table where he is already seated. She is still barefoot but he has shoes on.
Rebecca driving fast with her husband, hits Sam with such force that leaves him dead, but when it is shown them washing the blood off the buffer, not even a scratch is visible let alone a dent.
If early in the investigation the police had questioned the boyfriend who Britney claimed she had been staying with soon after she "disappeared", the entire ruse would have been exposed.