invalid cast in Page.LoadViewState

[Karl Seguin]

Our website occasionally throws the following exception:

at System.Web.UI.Page.LoadPageViewState() at
System.Web.UI.Page.ProcessRequestMain()
Specified cast is not valid.

So far no pattern or ability to reproduce at will. The error appears to
occur when a user visists a page (no postback, no complex parameters).

The site is running on a large web farm, and:
1 - EnableViewStateMac is set to false
2 - each machine has the same machine key.
3 - For the most part, viewstate is extremely small or disabled (except the
main page viewstate still shows up)

Any help in finding the cause would be appreciated
Karl