- "Holy Shit!" Quotient: Nobody saw that the third Holmes sibling would be a woman, let alone the true Big Bad of Series Four.
- Moral Event Horizon: Culverton Smith pretty much on everything he does considering he wants to kill just for the sake of wanting to murder "anybody" all while using his wealth and power to avoid getting caught.
- They Wasted a Perfectly Good Plot: We learn that Eurus has been posing as John's therapist; she was also the woman he'd been having a text-based affair with, and they met face to face then at least once. The following episode reveals that, unless you're a supergenius like Mycroft and Sherlock, she can implant orders into your mind just by talking to you. You'd think this would be set-up for her having some control over John. Nope!
YMMV / Sherlock S4 E2 "The Lying Detective"