Rollback doesn't preserve integrity of original index

After several "updateDocuments" calls a rollback call does not return the index to the prior state.
This seems to occur if the number of updates exceeds the RAM buffer size i.e. when some flushing of updates occurs.

Test fails in Lucene 2.4, 2.9, 3.0.1 and 3.0.2

JUnit to follow.