HttpRequest.Unvalidated Property

Provides access to HTTP request values without triggering request validation.

Use the HttpRequest.Unvaildated property to read the unvalidated form value.

http://msdn.microsoft.com/en-us/library/system.web.httprequest.unvalidated.aspx

Request validation checks for HTML markup and script that might indicate a potential cross-site scripting attack. By default, all values are checked using request validation and if any values contain markup or script, ASP.NET throws an HttpRequestValidationException exception. Use this method if you anticipate that the request will contain markup (for example, you are allowing users to post content that contains markup) and you want to get the raw value of a request.