Here was a post from another thread about the same issue:
"Regarding the error, a Google search found this comment:
"I'm quite certain this is generated by users on slower connections that click something on the page before the viewstate is completely loaded or is corrupt for some reason."
That would explain why I can't replicate the problem; I'm obviously on a fast connection to my server."
I can't get it to do it, but I'm on the same subnet as the server. I can't do it from home, because I'm on a fast DSL connection. I really think that this may be the issue though... it's very random, but it usually happens with "larger" orders, and with people who are physically/logically "far away".
Is there the equivelant of "Buffer = True" or something that can be done, so that the user can't do any more input until viewstate is completely loaded?