This error is really annoying and many of us face this quite oftenly. This may occur when you add a new item in Dropdown (with clientcode/ javascript) that is populated from server-side and postback occurs, binding a Grid and adding controls dynamically and clicking but my case was:

if page is not fully loaded and the _EventValidaiton hidden field is not present by that time then clicking a button or a postback was raising this exception. Simplest solution that I found after spending days was to install .NET framerwork 2.0′s SP2 which can be intsalled as part of framework 3.5 SP1 (besides writing this post I am also searching to get 2.0 SP2 as a standalone intsall). Well if thats the problem I hope you can solve this with SP2 intalled and can try methods listed here:

http://blogs.msdn.com/tom/archive/2008/03/14/validation-of-viewstate-mac-failed-error.aspx

After intalling I observed that noe EventValidation was rendering at top and controls after that. This fix ensures that EventValidation has the hash of every control before they are displayed.

Note: Many posts suggest to disable EnableEventValidation attribute of page directive but folks please try to avoid it as much as you can.