title: ASP.NET Invalid character in a Base-64 string
body: <p>I recently implemented ELMAH on my site and I have noticed that we frequently get the "Invalid character in a Base-64 string" error. I have never triggered it myself and none of our users have complained about it so I don't know what is going on. From the little I was able to find about it it seems like the viewstate might be getting too big or becoming corrupted or something. Does anybody know what causes this and how to prevent it? Here are what I believe are the pertinent lines in the YSOD.</p>
  
  <pre><code>[FormatException: Invalid character in a Base-64 string.]
  [ViewStateException: Invalid viewstate. 
  [HttpException (0x80004005): The client disconnected.]
  </code></pre>
  
  <p>Is there anything I can do about these errors or should I just filter them in ELMAH?</p>