Normal Maps Can't Be Mirrored

If you have ever textured an airplane, then you know that you can't use the same texture for both sides of the plane if there is writing on the fuselage. The writing will be horizontally mirrored on one side.

The same thing goes for normal maps - you can't mirror a normal map without getting bogus results. Think of your normal map as a real 3-d (slightly extruded) piece of metal. If you are seeing the text go the wrong way, the metal must be facing with the exterior side pointing to the interior of the airplane.

Having your normal map flipped does more than just "inset" what should be "extruded" - it completely hoses the lighting calculations. In some cases this will be obvious (no shiny where there should be shiny) but in others you will see shine when the sun is in a slightly wrong position.

The moral of the story is: you can't recycle your textures by flipping if you want to use normal maps.