Index "failed to merge"

Hi,

We're got the following problem on the 15th:
[2011-10-15 16:10:19,023][WARN ][index.merge.scheduler ] [Rand, Daniel]
[co0181ca0607][0] failed to merge
org.apache.lucene.index.CorruptIndexException: docs out of order (221 <= 221
)
at
org.apache.lucene.index.FormatPostingsDocsWriter.addDoc(FormatPostingsDocsWriter.java:84)
at
org.apache.lucene.index.SegmentMerger.appendPostings(SegmentMerger.java:590)
at
org.apache.lucene.index.SegmentMerger.mergeTermInfos(SegmentMerger.java:538)
at
org.apache.lucene.index.SegmentMerger.mergeTerms(SegmentMerger.java:470)
at
org.apache.lucene.index.SegmentMerger.merge(SegmentMerger.java:109)
at
org.apache.lucene.index.IndexWriter.mergeMiddle(IndexWriter.java:4273)
at org.apache.lucene.index.IndexWriter.merge(IndexWriter.java:3917)
at
org.apache.lucene.index.ConcurrentMergeScheduler.doMerge(ConcurrentMergeScheduler.java:388)
at
org.apache.lucene.index.TrackingConcurrentMergeScheduler.doMerge(TrackingConcurrentMergeScheduler.java:88)
at
org.apache.lucene.index.ConcurrentMergeScheduler$MergeThread.run(ConcurrentMergeScheduler.java:456)

After that, it's just a one line warning message ever so often.

We have 2 shards and one replica, so the data should be safe, right?

How do I fix this?

...Thanks,
...Ken