For example setting the default number of shards. This throws no exception, even though it is not supported.

CURL Apply change:
curl -XPUT localhost:9200/_cluster/settings -d '{ "persistent" : { "index.number_of_shards" : 2 } }'

Change Response
{"acknowledged":true,"persistent":{},"transient":{}}

CURL List settings
curl -XGET localhost:9200/_cluster/settings

Settings Response
{"persistent":{},"transient":{}}

Mailing list topic: https://groups.google.com/forum/#!topic/elasticsearch/uOXvRid-VoA