In ASP.Net, I sometimes get the following error – Viewstate verification failed. Reason: The viewstate supplied failed integrity check

Background: I developed a web app in ASP.Net 3.5 in C#. It runs great but now a few users have reported that they recieve an error sometimes. I have tried to duplicate the error but it has been hard. One time though when I let the app sit I came back later and tried to move it along and it errored out. I would think that has something to do with the timeout but I am using the view state.