We have a textbox inside an update panel and it postsback on text change. Around 1 in 1000 POST request to server is missing the __EventValidation form element ( we verified using the logs that, when this exception occured the request was missing this form element )
As this is not happening consistently, we are ruling out any coding issues. We suspect the user is posting the form even before the form is completely loaded.
Question: Is it ok, to move this __EventValidation element to the top section of the form instead of bottom if we are not doing any Response.Flush by overriding the Render method?
It looks like the .NET 2.0 SP2 has this fixed.
http://blogs.msdn.com/tom/archive/2008/03/14/validation-of-viewstate-mac-failed-error.aspx#8970568