I was doing an experiment with a backdrop and an infinite plane configured with a shader set to be a shadow catcher. I used a distant light to match the sun's position in the origional picture and adjusted the angle of the camera/plane to match the road surface in the image.

Because I had an anything glows object with a blue glow with shadows enabled, I got a bright orange "shadow" over the scene. Turning off the shadows for the anything glows object fixed that.

I believe that there could be a bug in the system here because that orange only affected the defined area of the "infinite" plane where as I'd expect it to cover the whole of that plane.

Does anyone know if I could use this effect to catch the blue lights of the ambulance? In the example below it might be nice if the light reflected off the taxi. I plan to use the ambulance with real objects rather than a backdrop so it's not an issue, I'm just interested to know.

http://www.youtube.com/watch?v=XAbCSJPIJwk

I don't believe an infinite plane can be used as a shadow catcher. It probably shouldn't work at all, but it's defaulting to working as a regular plane. I wouldn't call it a bug, but a nonexistent feature. Did you try using a regular plane?