I saw a documentary, I think it was about the Zodiac killer, where they interviewed a detective who had worked the Zodiac case back in 1969.
He said something like "in '67, you were supposed to come to San Francisco with flowers in your hair, but by '69, you would have been wise to also be carrying a revolver."
Good advice for visiting SF in 2017 as well. Also, if you park your car on the street, don’t leave anything inside the passenger compartment. Little piles of window glass on the roadway tell you why.