after same email on developer list:
"I developed a stress test to assert that a new document containing a
specific term "X" is always found after a commit on the IndexWriter.
This works most of the time, but it fails under load in rare occasions.
I'm testing with 40 Threads, both with a SerialMergeScheduler and a
ConcurrentMergeScheduler, all sharing a common IndexWriter.
Attached testcase is using a RAMDirectory only, but I verified a
FSDirectory behaves in the same way so I don't believe it's the
Directory implementation or the MergeScheduler.
This test is slow, so I don't consider it a functional or unit test.
It might give false positives: it doesn't always fail, sorry I
couldn't find out how to make it more likely to happen, besides
scheduling it to run for a longer time."
I tested this to affect versions 2.4.1 and 2.9.1;