System Information
Operating system: win 10
Graphics card: Geforce RTX 2070

Blender Version
Broken: 2.90.0 alpha
Worked: N/a

Short description of error
On my mesh , with custom normals, (i applied a withged normal modifier)
I had to clear the custom split normal date to get rid of artefacts given with the Ambiant Occlusion node in Cycles

Also seen with smooth shader:

Exact steps for others to reproduce the error
Another way to have this bug (without Custom Split normals):

• Create a suzanne.
• Smooth the normals
• Use Cycles render.
• Add an Ambient occlusion node and plug it on an Emit shader