Sitecore automatically creates dynamic fields for certain languages. A user might encounter the following error when indexing the language that is not available in the Solr schema:
99524 2015:06:25 14:52:03
WARN Crawler : AddRecursive DoItemAdd failed - {6601373B-C31A-43D7-8DD8-6429ACA38298}
Exception: SolrNet.Exceptions.SolrConnectionException
Message: <?xml version="1.0" encoding="UTF-8"?>
<response>
<lst name="responseHeader"><int name="status">400</int><int name="QTime">98</int></lst>
<lst name="error"><str name="msg">
ERROR:
[doc=sitecore://master/{110d559f-dea5-42ea-9c1c-8a5df7e70ef9}?lang=cs-cz&ver=1&ndx=sitecore_master_index] unknown field 'title_t_cs'</str>
<int name="code">400</int></lst>
</response>