When you have task sequences that uses MDT 2012 boot images they will no longer work after upgrading SCCM to 2012 R2.
The boot image will boot as expected but the task sequence ends unexpectedly at Setup Windows and ConfigMgr.
In the SMSTS.log you might see the error:
Failed to resume task sequence (0x800700EA)
You will need to use the default boot images or create new MDT 2013 boot images, so that they are updated to version 6.3.9600.16384 (ADK 8.1).