why abiant occlusion creates weird artefacts

I’ve got stange artefact using baked ambient occlusion

Without :

68552-2016-04-21-15-50-07-unity-personal-64bit-ambiantwh.png

with :

68553-2016-04-21-15-54-20-unity-personal-64bit-ambiantwh.png

not a reply but my actual parameters

The first rendering just set the Ambient Occlusion to zero