If you face such above mentioned error on inserting a value then that value might be containing anchor tags(< or >). If you want to insert these values you have to write
ValidateRequest="false" in page directive (first line of .aspx file)
Example:
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="xyx.aspx.cs" Inherits="xyz" ValidateRequest="false" %>
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