I've seen quite a few people complaining about this error and still haven't seen a good response. Most answers talk about machine keys, server farms, Server.Transfer, etc. However, the answer is actually very, very simple...
If a user stops a postback (usually by hitting the back button or stop button in the browser), not all the data is sent back to the server. When ASP.NET validates the viewstate, it fails.
I think the bug (if you can call it one) is actually in IIS, not ASP.NET. If the full postback isn't received, IIS shouldn't forward the request to ASP.NET.
Anyhow, the good news is this error doesn't affect the user's experience in any way. They navigated away from the page or stopped the postback and got exactly what they asked for. The server choked on the response, but that's exactly what it should do.
Chris
