Final Shot - S9-E3
Revealing mistake: As Reid explains human reaction time, near the end of it you can see a brunette woman get hit and fall to the ground, she hits ground with the back of her head and then, while supposedly been shot dead, moves her right hand in under her head.
Chosen answer: Back in Target Rich, a hitman named Giuseppe Montolo dies whilst being interrogated by Derek. The men who abducted Derek were hired by his father, Chazz, who seems to blame Derek for his son's death. It's explained further in A Beautiful Disaster.
Annabel Keeley