Customers running Windows Server 2012 and 2012 R2 on top of any iSCSI based storage solution that uses multipathing should URGENTLY install the latest hotfix from MS to be found here. The reason behind is a critical bug is an error that causes data corruption after a path failover takes place. The available information are quite short, here is the official text from the kb article:
"If the connection to an iSCSI array is disconnected because of a certain issue, one or more iSCSI sessions or paths to the array fail. In this situation, Windows iSCSI initiator recovers the connection. However, the recovery causes data corruption."
Doesn't explain too much around the background of this problem but the bug seems to be serious. As already said a hotfix is available and all customers running an affected scenario are asked to update their servers as soon as possible.
As we have most of our installed base running on FC or using VMware as hypervisor between the Windows systems and the storage, we aren't affected until now. Even some customers running Hyper-V on top of DataCore based iSCSI storage haven't reported any problems although they definetly did path changes in the recent past during maintenance windows. So I can't confirm that this issue is 100% happening but just to be sure plan to install the hotfix.