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

You may see this error when you are insert html in asp.net forms. To fix this just append this at the top of page

ValidateRequest="false"

But if you are using dot net framework 4, you also need to add this in the web.config file

<httpRuntime requestValidationMode="2.0"></httpRuntime>

and you are done.

For selected folders use "location" object inside folder and place above code inside that

<location path="{path}">
    <system.web>
        <httpRuntime requestValidationMode="2.0" />
    </system.web> 

</location>