We would like to draw your attention to a bug in SQL 2012 (RTM, SP1 as well as SP2) as well as SQL 2014, which may result in corrupt data when running index rebuilds online (SQL Enterprise only or equivalent) under certain circumstances (including multi-threaded rebuilds).
More information about this and a hotfix for SQL 2012 SP1 and 2012 SP2 is available at http://support.microsoft.com/kb/2969896. Unfortunately, the fix is not included in CU10 for SQL 2012. A temporary solution is to enable MAXDOP=1 on the indexing job, or choose to run reorganize instead of rebuild.
The fix is coming to all versions except SQL 2012 RTM, as this is no longer patched by Microsoft.