The fact that Lt. Monahan bought a man he assumed killed the young woman in the house on the beach to Vickie's home to identify is beyond unprofessional and dangerous. Police lineups are done behind one-way mirrors for a reason.
When Leon attacks Vickie with her back towards him, it would have been impossible for her to have stabbed him in the chest while holding the scissors with the pointed side below the heel of her hand.
When Sam makes an impression of the gold chain necklace Quincy uses a magnifying lens to notice specific identification markings from it, but when the actual necklace is shown to him he instantly recognizes those exact same markings with his naked eye.