This issue happening if we entered text in controls that look like HTML statement. We can omit this issue  to set validation request is False.
<%@ Page Language="C#" MasterPageFile="~/ControlPanel/Site1.Master" ValidateRequest="false" AutoEventWireup="true" CodeFile="Cookies.aspx.cs" Inherits="ControlPanel_Cookies" %>
We can set this settings in globally when we write like below in web.config
<%@ Page Language="C#" MasterPageFile="~/ControlPanel/Site1.Master" ValidateRequest="false" AutoEventWireup="true" CodeFile="Cookies.aspx.cs" Inherits="ControlPanel_Cookies" %>
We can set this settings in globally when we write like below in web.config
<system.web><pages validateRequest="false" /> 
