A potentially dangerous Request.Form value was detected from the client

It means that you can't post values containing HTML ( or script ) tags to the server. In other words - if you have a textbox and the visitor enters something like
< script> alert('a') < "/ script> and then presses Submit button this error will occur since the posted value(s) contains HTML tags.,it's for security reasons.to prevent this you can add a tag validateRequest="false" into the Page-directive on top of the page you want users to be able to input HTML and script tag or you can write in the web.config fileit will affect all pages on your site.You can read more at ASP.Net site.