Microsoft has retracted the Service Pack 1 for SharePoint Server 2013, SharePoint Foundation 2013, SharePoint Designer 2013 and Office Web Apps Server 2013. The reason for this move is stated by Microsoft in the knowlege base article:
We have recently uncovered an issue with this Service Pack 1 package that may prevent customers who have Service Pack 1 from deploying future public or cumulative updates.
If you have already downloaded the Service Pack 1 for SharePoint 2013, it is recommended you wait with the installation for an updated version of the Service Pack 1.
If you have already installed the Service Pack 1, you should not expect any problems. However Microsoft will release a patch to fix the problem in your Service Pack 1 version. It is recommended to wait for this fix before your install any other patches. Bill Baer has a post that covers this issue.
SharePoint 2013 with SP1 also known as Slipstream installations are not affected by this issue.