Windows 7 64-bit, GTX-580, AMD FX-4100, 8 GB RAM
Broken: Buildbot 2.71 698a9a2, and buildbot 2.71 b49e6d0
Worked: 2.70a official
Short description of error
On an object with negative scale, a cycles Fresnel or Layer Weight (just Fresnel output, not Facing) produces a sharp divide in final (but not viewport) renders, instead of a smooth gradient, as seen below:
The sphere on the left has x-scale -1; the one on the right is all positively scaled.
Exact steps for others to reproduce the error
Take some smoothly-curving object, scale it by -1 along some axis (or all three, but not just two, that isn't broken;) and apply a material that uses a Fresnel mixing factor (default IOR/mix factor.) There will be a sharp divide where there should not be on the final render, but not in the viewport render,
Alternatively, see here:
Try the two different Fresnel nodes and the Layer Weight, and observe that only Layer Weight works in final renders, but all three work in the viewport.