When I was trying to recover from our bad redundancy I noticed that CirrusSearch would sometimes continue when Elasticsearch failed to apply some change. I know this error occurred for applying mappings. Here is an example:
Validating mapping for page type...different...failed!
Couldn't update mappings. Here is elasticsearch's error message: RemoteTransportException[[testsearch1001][inet[/10.64.32.138:9300]][indices/mapping/put]]; nested: ProcessClusterEventTimeoutException[failed to process cluster event (put-mapping [page]) within 30s];
Validating aliases... Validating content alias...is taken... Reindexing...
So far as I can tell the Elasticsearch error was caused by the mixed versions on the Elasticsearch cluster bug but it should have caused Cirrus to stop cold rather than proceed.