This particular nugget has screwed with me for a few days. When attempting to run a standard EPiServer Find index, my log files were getting flooded with this:
An exception occurred while indexing (Content): MapperParsingException[failed to parse [Blocks.Items.Blocks.Items.DefaultPrice.UnitPrice.Currency]]; nested: ElasticSearchIllegalArgumentException[unknown property [CurrencyCode$$string]]; .
Contact episerver support to get them to move your index to a new cluster. In my case it was confirmed by EPiServer support that it was a timeout issue with regards to a bad cluster the index is sitting on.