AssertionError triggered in IndexWriter.filesExist [LUCENE-4699]

Occasionally I get this AssertionError in my unit tests:

java.lang.AssertionError: file _0_Lucene40_0.tim does not exist
at org.apache.lucene.index.IndexWriter.filesExist(IndexWriter.java:3981)
at org.apache.lucene.index.IndexWriter.startCommit(IndexWriter.java:4047)
at org.apache.lucene.index.IndexWriter.prepareCommit(IndexWriter.java:2800)
at org.apache.lucene.index.IndexWriter.commitInternal(IndexWriter.java:2875)
at org.apache.lucene.index.IndexWriter.commit(IndexWriter.java:2855)
at org.apache.lucene.index.IndexWriter.commit(IndexWriter.java:2839)

I'm not sure what's supposed to trigger this. To fix this I just rerun my tests and the error is gone.

I believe it's probably caused by something I'm doing wrong, but I was hoping to get some kind of declared exception and not an assertion error.

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Migrated from [LUCENE-4699](https://issues.apache.org/jira/browse/LUCENE-4699) by Raimund Merkert

Start with IndexWriter.filesExist and the commit path shown in the stack trace; reproduce the intermittent failure in the unit tests and inspect why _0_Lucene40_0.tim is missing. Compare the observed behavior with the requested declared exception, and document a reproducible trigger and expected handling.

Written by the indexing model from the issue text.