You'll get this error from the .Net framework when it detects potentially harmful content in a web request submission. Want to disable this validation? Read on...
For normal ASP.Net Web forms apps you can disable request validation by setting validateRequest=false in the page directive (top of your aspx page) or in the configuration section.
For ASP.Net MVC apps add the [ValidateInput(false)] attribute to the appropriate controller action.
Be careful when turning off this value as you are making your site vulnerable by accepting potentially harmful input. You should explicitly validate your input in your code if you disable the built in .Net request validation.