POST _ingest/pipeline/_simulate
{
  "pipeline": {
    "processors": [
      {
        "lowercase": {
          "field": "test"
        }
      }
    ]
  },
  "docs": [
    {
      "_source": {
        "test": [ "FOO", "BAR" ]
      }
    }
    ]
}

{
  "docs" : [
    {
      "error" : {
        "root_cause" : [
          {
            "type" : "ingest_processor_exception",
            "reason" : "java.lang.IllegalArgumentException: field [test] of type [java.util.ArrayList] cannot be cast to [java.lang.String]"
          }
        ],
        "type" : "illegal_argument_exception",
        "reason" : "field [test] of type [java.util.ArrayList] cannot be cast to [java.lang.String]"
      }
    }
  ]
}