Sitecore 7: Exception of type ‘Lucene.Net.Index.MergePolicy+MergeException’ was thrown.

Intro

Recently I got this  error, Exception of type ‘Lucene.Net.Index.MergePolicy+MergeException’ was thrown, in our log files. This was a Sitecore 7 site, rev. 140905.

First try and error punsh was to delete the files in the indexes folder ("sitecore_master_index") and rebuilding the index. Suprisingly the issue went away. Rebuiling usefull on a dev machine...

Here the error log entry:

Lucene Merge Thread #0 14:54:38 ERROR Unhandled exception detected. The ASP.NET worker process will be terminated. 

Exception: Lucene.Net.Index.MergePolicy+MergeException

Message: Exception of type 'Lucene.Net.Index.MergePolicy+MergeException' was thrown.

Source: Lucene.Net

   at Lucene.Net.Index.ConcurrentMergeScheduler.HandleMergeException(Exception exc)

   at Lucene.Net.Index.ConcurrentMergeScheduler.MergeThread.Run()

   at System.Threading.ThreadHelper.ThreadStart_Context(Object state)

   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)

   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)

   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)

   at System.Threading.ThreadHelper.ThreadStart()