Occasionally, users on our Kentico built website get the following error:

Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that <machineKey> configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster.

The page itself just has a simple form which collects some user details, and on submit stores them into a database - this error happens very randomly. I also checked the value of <input type="hidden" name="__VIEWSTATE" id="__VIEWSTATE" value="..."/> and it is consistent between IISresets and apppoolrecycles

Anyone know why users are getting this error?

Done quite a bit of googling (eg http://www.appetere.com/Blogs/SteveM/July_2009/Working_with_ViewState.aspx) but if anyone has any ideas would be much appreciated :)