You may see a "The URL-encoded form data is not valid" error. while posting a large amount of data to the server.
This error is a side effect of a security fix released by Microsoft (to read more about this issue, see this link: http://weblogs.asp.net/scottgu/archive/2011/12/28/asp-net-security-update-shipping-thursday-dec-29th.aspx)
In a nutshell, there is now an upper bound on the number of simultaneous HTTP form elements that may be posted.
The default is now 1000 without explicitly changing it with this key in the <appSettings> portion of the web.config:
<add key="aspnet:MaxHttpCollectionKeys" value="2000" />