Custom Diffuse CG shader from wiki strange behaviour

Hi,

I recently tried to make a custom shader with cg which requires the use of unity lights and to achieve this i based my shader on the diffuse shader available on this wiki :

http://en.wikibooks.org/wiki/Cg_Programming/Unity/Smooth_Specular_Highlights

But then i found that this shader code was giving me unexpected strange results with point lights like here:



I don't know if this is a normal stuff and the result i should expect or if i may do something wrong somewhere.

Thank you for your help.