Microsoft have just posted a hotfix for a very serious bug in SQL Server 2012. 

This bug could potentially cause you to lose data during online index rebuilds.

At this time I would not recommend downloading the hotfix (wait for a CU which will address the problem). In the meantime follow the posted workaround which is to perform online index rebuilds with a MAXDOP of 1. For example:

ALTER INDEX <idxname> ON <objectname> REBUILD WITH (ONLINE=ON, MAXDOP=1);