I was printing sun.stl at: http://www.thingiverse.com/thing:34778 . It has a concave section on one side, which was down because the other surfaces are not flush with the bed. I previewed the gcode. On one layer, and only one, the concave area of the layer was shown in yellow/orange. What does this mean?
I thought it may have been trying to warn me that the area was in danger of sagging, which it did. I checked the box for scaffolding (support) and tried again. The colored area was still in the preview, but the build was OK. The original model is manifold.