Character mistake: In this episode the unsub is copying the murders of Jack the Ripper (with men as victims). The Unsub also taunts the police and BAU with letters, even using the Dear Boss salutation. The BAU draw upon the Dear Boss letters from the original Ripper case as part of their profiling - not only of the unsub but also the Ripper. Handwriting comparisons has shown that the author of the Dear Boss letters was most likely a journalist and not the Ripper. Whilst the unsub may have been ignorant of the forgery status of the dear boss letters, you would expect the BAU to be wiser.
Factual error: Reid says that it's been speculated that Mary Jane Kelly was murdered in a flat rented by the killer - this is completely incorrect. She was murdered in her rooms, where she'd been living since early 1888.
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