I am so glad you responded to questions today because I came up with a big one yesterday. I was on an F heading up market around 3rd when a regular bus in the opposite direction passed us with the sign saying "EMERGENCY CALL 911". I looked around and no one else seemed to care about this. I've never seen a bus with this sign before so I thought it might mean 'this bus is having an "EMERGENCY" please "CALL 911"'. I know lame, but i thought it might be a way that the driver can alert others without drawing suspicion to whomever on the bus is causing trouble (but i assume they have some other way like a panic button). I didn't call 911 because I figured my muni driver would've done something if it'd mattered, but please tell me what the hell is this sign and what is it for? If its some way of remininding pedestrians "if you have an emergency, please call 911" then i think its a horrible way to advertise it.

This is a way for the drivers to be discrete and have the police notified. Please call it in. Remember the number on the bus, coach, trolley - the direction of travel, where you last saw it and any odd observations you may have seen.

Emergency? Doubt it. It's more than likely just some MUNI ineptitude.
See also:
http://www.flickr.com/photos/19417968@N00/273582229/
http://www.flickr.com/photos/19417968@N00/881294951/
http://www.flickr.com/photos/19417968@N00/791638545/

No doubt, but we wish to have these reported, just in case it is the real deal.