While working with ASP.NET 2.0, if you try to put some XML text in a textbox or an invalid email in a textbox, then you get following error, when you submit the page. This occurs becuase the request is being validated by the server when you submit it. To avoid this error, you have to add following line to your web.config file inside the section :
So, when you get this error, try this solution.
If you turn off validation, you MUST apply some sort of filtering to any page that accepts input (or better yet, to all pages). Here's a topic in the documentation that suggests one way to create a simple whitelist filter:
How to: Protect Against Script Exploits in a Web Application by Applying HTML Encoding to Strings
More info about what ValidateRequest does: