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.
At least one time in you life you see this message. And this can be a real problem... It's infrequent and hard to reproduce this error.
They are saying something about cluster? I'm on shared hosting... So, why this problem occurs?
It's hard to find why this error occurs. But let me explain why. IIS can shutdown your application if there are no request during some time... And this is because someone just fills big form! (and no other visitors this time). And when someone submits a long form, he receives this, message. I don't know, maybe this was a problem only for me. But solution is quite simple! Always use generated <machineKey>. There are another question... How to generate it? I've found a tool long time ago...
So, use MachineKey generator http://www.aspnetcafe.com/tools/machineKey.aspx
Maybe this stuff can save you several hours... usually, for beginners in ASP.