After reloading a core, indexing will fail on any newly added fields to the schema.

This is just another instantiation of SOLR-3587.

Because the IndexWriter is kept open, the Codec still uses the old schema, and when asked to index for the field, doesn't know about it and returns an error.

We could dodge the problem for now by turning off per-field codec support by default, requiring users to turn it on explicitly in solrconfig.xml (like similarity), knowing they are making a tradeoff by doing that (same goes for Similarity if you are using a per-field configuration).