Strictly a storage (or storage virtualisation) KB article, but relevant to Hyper-V admins, this post describes when data corruption occurs when you merge Data Deduplication-enabled VHDs in Windows Server 2012.
Assume that you enable Data Deduplication on a virtual hard disk (VHD) on a Windows Server 2012-based computer. The computer crashes while you are merging VHD files. In this situation, data corruption occurs in the VHD files.
To resolve this issue, install the Windows 8 and Windows Server 2012 cumulative update 2822241 (available via Windows Update).