The giant flag on the corner of Castro and Market is at half staff today. I don't know why and neither did others I talked to this morning.
The flag is black, blue and white with a red heart on it. The flag at this site is usually a rainbow colored one, but several times a month it is this flag, the flag of the SM (Sado-Masochism) sub-community.
I doubt that this public symbol would be used for humor, so I assume the half-staff refers to a prominent community member of who died recently.