There's something I don't understand (may be an early morning issue); sometimes I want an object to be mirrored, so I negate the X or Z. Consequently, normals should be flipped, but it seems that is not always the case. Yesterday I rendered something that way, but the object was 'negative' (dark, with light on the wrong side), so I undid the flipped normals, while the Z was still a minus 1. Rendered again and it was okay. Mmmm.
This morning I did a little test. This time, as expected, with the Z=-1 and flipped normals it rendered good, but didn't with non-flipped normals (and Z=-1). Either in normal render mode, defer mode or PT. The RTP however looked okay, whatever I changed, thus contradicting the final render.