If you see this error in your Arnold log, it most likely means that you have more than 256 per-face shaders. Arnold has a hard limit of 256 shaders that can be assigned to a single object.

[polymesh] /pPlane1/pPlaneShape1: out-of-range shader index (5/3)

You may also see this warning if you have a large number of objects merged into one single object (for example, an Alembic archive that consists of many objects). In some cases, an Alembic archive like that may have more than 256 shaders assigned to the sub-objects.