ParallelTermEnum.next() fails to advance properly to new fields. This is a serious bug.
Christian Kohlschuetter diagnosed this as the root problem underlying
LUCENE-398 and posted a first patch there.
I've addressed a couple issues in the patch (close skipped field TermEnum's, generate field iterator only once, integrated Christian's test case as a Lucene test) and packaged in all the revised patch here.
All Lucene tests pass, and I've further tested in this in my app, which makes extensive use of ParallelReader.