When causing a refresh of Solr bundles, write locks can sometimes be left around. This can be reproduced reliably by restarting the `Solr Server` bundle and then the `Solr Banana Provisioner` bundle. It appears that restarting the DDF may cause the exception to go away, but there are other Solr exceptions on startup, so it may just be masked.
This could be fixed by searching for and deleting write lock files when trying to instantiate cores.
See attached stack trace for more info.