Much to Terry Child's dismay, there will be a ShotSpotter test tonight in the Mission. From the authorities at SFPD’s Mission Station (via Mission Mission):

Please Note: On Thursday, December 18, 2008 testing of the gunshot recognition/locator system known as “Shotspotter” will take place in the Mission District. It is anticipated to occur at approximately 9:00 PM and will be occurring at various locations. The testing will occur in the areas of 18th St & Mission, 22nd St. & Valencia, 23rd St and Folsom, and 18th St and Bryant. Please do not hesitate to call 911 if you become aware of suspected gunshot sounds at any other time at those locations.

Which is to say, you might hear loud booming sounds tonight, Mission residents. You've been warned.