Here's how you can replicate this issue.
- Modify a single item such that it has two "dc.date.accessioned" values.
- Restart Tomcat/Solr (may not even be necessary)
In the Tomcat logs you will see errors like:
Aug 17, 2011 12:54:18 PM org.apache.solr.common.SolrException log
SEVERE: org.apache.solr.common.SolrException: can not sort on multivalued field: dc.date.accessioned_dt
This error seems to cause the Item to no longer be available from Discovery or the UI in general.
Although most items in DSpace should only ever have one 'dc.date.accessioned', it still is not impossible to have multiple values. If you import any item which already has an existing 'dc.date.accessioned' value (for whatever reason), DSpace will keep that value & also add a new one representing the new DSpace Accession Date (see the InstallItem class).
In general, if possible, it'd be best for Discovery to not assume a single value for any 'dc.date' fields.
This same issue also occurs if you manually add a second 'dc.date.issued' value. You'll see similar errors stating "can not sort on multivalued field: dc.date.issued.year".