4. The Queen Smokes
This is definitely one of the more zany deductions in Sherlock. In the first episode of the second series Sherlock and Watson are summoned to Buckingham Palace by Mycroft Holmes to aid in the acquisition of lost state secrets. Sherlock memorably refuses to dress as Mycroft will not tell him who the client is. Even more memorable though, is the deduction that Sherlock carries out whilst sitting in the Palace. In short, the Queen smokes. He notices the ashtrays around the place and realises that the Queen's chief of staff does not smoke. This leads him to deduce that it must be Her Majesty herself who has the smoking habit. This is certainly one of the more amusing Sherlock moments!