Revealing mistake: When Molly's phone is ringing, the display shows the call has already begun.

Continuity mistake: After Sherlock repeatedly tells Culverton that he doesn't want to die, in the closeup Culverton increases the dosage with his right hand, but when it cuts to the wideshot he's doing it with his left hand.
Super Grover
Chosen answer: That line of dialogue has never actually been spoken in any of the Sherlock episodes, during seasons 1-4. However, that line is written in someone's fan-fiction story online, where it's said by Sherlock and directed at Anderson.
Super Grover