When our team began testing our first .Net web application on a web farm, we came across the error:

The View State is invalid for this page and might be corrupted.

We quickly learned that the cause of the error was resulting from the fact that the machine keys in the machine.config file did not match on every server thus generating the error (see KB313091 and KB312906 on how to create the keys). So, it stands to reason that when we rebuilt one of our web servers this past week, all I had to do was make sure the machine keys matched up for the newly rebuilt server and I wouldn't have a problem. Wrong. Apparently, I forgot to install .Net Framework Service Pack 2  which also created the same error. I can't explain it, but by installing the service pack, I'm cooking with gas and the problem is resolved.