When an index is used over NFS, and, more than one machine can be a
writer such that they swap roles quickly, it's possible for the index
to become corrupt if the NFS client directory cache is stale.
Not all NFS clients will show this. Very recent versions of Linux's
NFS client do not seem to show the issue, yet, slightly older ones do,
and the latest Mac OS X one does as well.
I've been working with Patrick Kimber, who provided a standalone test
showing the problem (thank you Patrick!). This came out of this
Note that the first issue in that discussion has been resolved
LUCENE-948). This is a new issue.