Pics of the meltdown at Church/Duboce, with shuttle buses picking up frustrated passengers this morning. Why did it happen? Your guess is as good as Muni's. But according to Jameth:

All outbound N service is stopped here and trains are diverting along the J line. Shuttle bus service is active between here and Ocean Beach. I see an ambulance at the Dubce Park tunnel entrance. Inbound J service is also disrupted; service vehicles are blocking the tracks on Church just north of Market ... [P]eople are freaking out on the street. It is like Chinatown where everyone thinks OMG IT IS THE LAST 9x-Bayshore/30-Stockton/etc. EVER IN EXISTENCE and they all squeeze on.

Hope no one was killed or seriously injured. We'll let you know more when we do.