I had a fairly complex scene going and and I wanted to do basically this:

camera {<0,10,-10> look_at <0,10,0>} //looks forward along z axis
sphere {<0,9,-5>,.3 scale <0,0,2>}

that is, scale a sphere to double its original size in the +z and -z
directions,
making an egg sort of shape.  It was to be used as an endcap to a cylinder
going from 5 to -5 along the z axis. 
-I checked the sphere coordinates to make sure I wasn't making it at +z
instead
of - z.
-I checked the camera coordinates.
-I checked the scale syntax to make sure I wasn't scaling just the texture.
-I checked that the sphere's diameter was large enough to see.
-I made it a bright red pigment to make sure it wasn't getting lost in the
background.
-it hadn't been translated in place, it was at the coordinates described.

basically if I erased the scale <0,0,2> the sphere appeared where it ought
to be just fine.  If I put it back, it disappeared from the scene.  I think
I ended up making a larger one and flattening it along the y axis.

But I'm still curious as to what happened.

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