Strange issue with fresnel


I am having a really weird issue using a fresnel node to mix two shaders. Here is a brief description. I started to model a plane, and used a mix of diffuse and glossy balanced by a fresnel input node. Everything looked fine, until yesterday evening. I tweaked the mesh further and further and at a certain point I got this problem:

As you can see the fresnel effect is perfectly sharp. There is a solid edge between the diffuse and the glossy shader. This didn’t happen before the last save. The problem is present if I use a fresnel or a layer weight with fresnel, while using the facing output of the layer weight it works fine.

I tried to cancel the material and make a new one, or import the model into a new blend file, but the problem is still there. Looks like is directly connected with the mesh, but I cannot understand why…

Does anyone have a clue of what is going on?