After Hayley shoots and kills Wesley, she sits on the floor near him. Wesley's arms are out to his sides. The camera switches away then back. Now Wesley's arms are crossed over his chest. Hayley is still in the same position and there was no time for her to have crossed his arms.
Medical examiner Dr Parish states that a victim was stabbed in the back between the T3 and T4 thoracic vertebrae and "died instantly". Severing the spinal cord at that point could paralyze the chest and diaphragm, causing death by suffocation, but not "instantly".