»Uh-oh, I got a little problem.

An F-Market rolled down Church Street today, probably heading back to the Geneva / San Jose yard for the night, and tore through a line of cars. Since the car was heading back home, it probably didn't have any passengers.
This probably blocked J-Church traffic heading on the same line -- MUNI does not have a crossover that would permit the outbound J to travel the inbound tracks for the stretch along Dolores Park.

The wire report read:

Out-of-Control Streetcar Leaves Three Injured

San Francisco Muni has suspended the driver of streetcar that rolled out of control Thursday night.
The driver apparently stepped away from the antique car at a Castro Street stop, and the car slipped away, rolling down to Church Street. About five cars were hit, and three people were hurt.

Muni will reevaluate the driver's status after a full investigation.