Those that regularly download from the TechNet or MSDN sites are familiar with the issue when sometimes Microsoft's File Transfer Manager (FTM) will deliver corrupted files, forcing a redownload.
But, in the case of the slipstream ISO delivered for SQL Server 2012 SP2 to TechNet and MSDN subscribers recently, this is not the case, and you can stop trying to redownload because there is an actual issue with the ISO.
The problem? Microsoft posted an ISO that does not actually include SP2. Instead, when installed, it only installs the RTM version of SQL Server 2012. You'd think this was a unique situation, but unfortunately the same thing happened when the company supplied the ISO for SP1.
Microsoft is aware of the problem and is working on a fix. In the interim, if you want to walk through manual steps of creating your own, good version of an ISO that includes SP2, Microsoft is suggesting you use the same procedure the company posted as a workaround for the SP1 ISO.
Or, you can just install SP2 from the Microsoft Download Center.