Using enableViewStateMac requires that successive requests be forwarded to the same server (i.e. server affinity). This feature is used to help prevent tampering of a page's view state; however, it does so based on an auto-generated validation key on the current server. From this key, a message authentication code (MAC) is generated and sent in the ViewState back to the browser. The problem is that if a POST back is performed and goes to a different server, you will get a nice little error message saying “Corrupt View State“.
To fix this, you can either set enableViewStateMac to false in the <pages> element or specify a common value for the validationKey attribute in the <machineKey> element across all servers (in the farm).
By the way, documentation says that this is OFF by default. That is incorrect! Go check machine.config!
<!-- pages Attributes:
enableViewStateMac="[true|false]" // Default: false <- Even the comment says that it is false.
Visit http://msdn.microsoft.com/msdnmag/issues/03/02/CuttingEdge/ for more details.