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.
I am using localhost btw.
By default, ASP.NET will attempt to validate the viewstate. If validation fails, it will throw this exception. Reasons that it might not validate include recompiling the site and then refreshing a form in your browser, or some sort of server farm/cluster (but if you're using localhost, I'd lean towards the former).
You can turn this off if it's really hampering you, but then anyone can monkey with your viewstate. This isn't a big deal so long as you don't trust user input to begin with (and you shouldn't). In your web.config:
<pages enableViewStateMac="false" />
Check out this long thread on the official ASP .NET forum - it pretty much covers all the reason why this can occur and some solutions.
Note the following can often be a tempoary fix, but please be aware of the security implications of doing this.
<pages validateRequest="false" enableEventValidation="false" viewStateEncryptionMode ="Never" />
It may be to do with app recycling. Viewstate is signed by a key which is generated when your application first starts. If your application pool has recycled between requests then the viewstate signing key will change and the previous viewstate will be invalid and you will see the exception.
In order to rule this out you can set a specific machine key in your web.config.
One other thing of note is that if you have a page that makes heavy use of data binding your viewstate may be very large and the page may render to the browser before it's complete, and if you then do something that causes a post back before the page is fully downloaded you may see the error. This was fixed in .NET 3.5 SP1. You can also fix this by rendering the form as disabled then enabling it client side or by changing where viewstate gets generated. The ASP.NET debugging blog has the instructions.
If it still happens you're not attempting to make changes to the viewstate form field with javascript are you?