Ok guys... Some others renders :
First, same scene, no softshadows, sun at 40° elevation, two centered objects with reflective shader (raytrace and no raytrace):
No bug.
Same scene, softshadows turned on (1°, 20 samples), centered objects with reflective shader (raytrace and no raytrace):
Bug.
Same scene, softshadows turned on (5°, 20 samples), centered objects with reflective shader (raytrace and no raytrace):
Bug.
Now an other scene, same global settings, same nodes configuration (defaut shader -> reflective shader -> multishader -> object), but external nodes this time, no imported object but 2 spheres:
One sphere with reflective, simple shadows: No bug; one sphere with reflective, soft shadows: No bug; one sphere with reflective and ray traced option, soft shadows: No bug; two spheres with reflective and ray traced option, soft shadows: No bug; two spheres with reflective and ray traced option, soft shadows (more samples, more degrees): No bug. Can't reproduct it.
So I think bug appear only with reflective shader on imported objects, maybe just my imported object, maybe just on my computer. Some others renders are running with an other imported object. Is there a way to product same effect (raytraced reflection on a sub-object material) with an other nodes configuration?