Elasticsearch silently ignores mis-structured mapping

bad-mapping.sh

curl -XDELETE localhost:9200/mytest

curl -XPUT localhost:9200/mytest

curl -XPUT localhost:9200/mytest/t1/_mapping -d '{
"t1": {
  "_source": {"enabled": false}, 
  "properties": {
    "_all": {"enabled": false},
    "field1": {"type": "string"},
    "field2": {"type": "long"},
    "_underscore1": {"type": "string"},
    "_underscore2": {"enabled": true}}}}'

curl -XGET "localhost:9200/_mapping?pretty"
# will yield:
#{
#  "mytest" : {
#    "mappings" : {
#      "t1" : {
#        "_source" : {
#          "enabled" : false
#        },
#        "properties" : {
#          "_underscore1" : {
#            "type" : "string"
#          },
#          "_underscore2" : {
#            "type" : "object"
#          },
#          "field1" : {
#            "type" : "string"
#          },
#          "field2" : {
#            "type" : "long"
#          }
#        }
#      }
#    }
#  }
#}