**System Information**
Operating system and graphics card
Linux Mint 16 KDE x86_64
**Blender Version**
Broken: 2.68a-2.70 (earlier versions was not tested)
**Short description of error**
Dark areas on object while rendering with Cycles and using "Normal Map" node.
**Exact steps for others to reproduce the error**
1. Unwrap a model;
2. Mirror an UV island in UV/Image editor (mirroring in both axis gives more darkness);
3. Add "Normal map" node to "Image texture" (there's no really need to open an image);
4. F12.