Sometimes when Solr is restarted, I get some "lock held by this virtual machine" messages in the log, and the admin UI has messages about a failure to open a new searcher. It doesn't happen on all cores, and the list of cores that have the problem changes on subsequent restarts. The cores that exhibit the problems are working just fine – the first core load is successful, the failure to open a new searcher is on a second core load attempt, which fails.
None of the cores in the system are sharing an instanceDir or dataDir. This has been verified several times.
The index is sharded manually, and the servers are not running in cloud mode.
One critical detail to this issue: The cores are all perfectly functional. If somebody is seeing an error message that results in a core not working at all, then it is likely a different issue.