Microsoft has recently changed the behavior of the EnableViewStateMac setting. This setting allowed to control whether the ASP.NET Viewstate of the pages should be encrypted or not.
From September 09, 2014, ASP.NET Viewstate is always encrypted regardless of the value set for EnableViewStateMac.
For more information about this change, see the MSDN blog post:
http://blogs.msdn.com/b/webdev/archive/2014/09/09/farewell-enableviewstatemac.aspx
This change may cause inconsistent behavior of load balanced Sitecore sites.
Sitecore XP 7.5 Initial Release and earlier Sitecore versions contain pages that have the EnableViewStateMac setting set to false:
After the change is introduced, these pages will behave as if the EnableViewStateMac setting is set to true. This is not expected to cause any problems.