Because some idiot thought it would be fun to make ASP.Net websites crash randomly. This ill-designed "protection" is causing more harm than good in each and every website where it's not turned off. And now it seems in ASP.Net MVC 2 it can't be turned off. I've already modified web.config and this idiotic message still appears. Hey dude, I ain't ever gonna include that value unescaped in the HTML, shut the heck up!