Odd STL result, but I don't know why So I drew a threaded cap.  This is my first attempt.  It looks OK in Blender but when I view the resulting STL file in an STL viewer, it looks very wrong and I don't know where to begin to fix it since it already looks ok in the modeler.
Ok, I figured it out.
When you HIDE an object and forget that you hid it... It will be there in your export laughing at you.  Don't be like me... remember your hidden objects.