We encountered a few cases where trying to update a node via the REST API failed with a generic error like " {"error":{"code":"unknown","message":"Unable to perform operation. Node is protected."}}". Those cases were usually when trying to write a JCR interal property like "jcr:mixinTypes".

It would be helpfull if either the error messages were a bit more specific or if the documentation would mention operations which usually lead to such obscure errors