I remember leaving it very late to book a hotel for JavaOne once and being put in a hotel in what turned out to be a rather "interesting" location. My first hint that something might be up was when the taxi driver at the airport took one look at me and asked "Do you really want to go there?".
The second hint that things were going to be interesting was when the guy on the front desk told me how to get to Union Square - the hotel was on Geary St. and I thought I knew SF reasonably well. He proceeded to tell me not to walk directly along the street, but to walk up the hill a bit then walk across in the direction of Union Square.
It turned out that during the day this advice wasn't really required but at night it definitely was! I only walked back once and took taxis back after that.
This was about 10 years ago - don't know if the area is more civilised now but it was pretty scary then.
