There is a race condition if one machine is checking the current version of an index while another wants to update the segments file in IndexWriter.close().
java.io.IOException: Cannot delete segments
On the windows platform reading the contents of a file disallows deleting the file.
I use Lucene to maintain an index of +-700.000 documents, one server adds documents, while other servers handle the searches.
The search servers poll the index version regularly to check if they have to reopen their IndexSearcher.
Once in a while (about once every two days on average), IndexWriter.close() fails because it cannot delete the previous segments file, even though it hold the commit lock.
The reason is probably that search servers are reading the segments file to check the version without using the commit lock.