Camera coordinate appears to be left handed It is weird that I have a camera that has a left-handed coordinate system.
All the other cameras are right handed. 

Left handed camera:


Right handed camera (all others are right handed, too):


I can't tell what caused this.
As hinted at by cegaton, you set the x-dimension of the scale to a negative value for one of the cameras. Open the properties region with N and set the scale back to positive 1 if you like.