Recently I inherited a ASP.NET project from a colleague, who left the company. Call it Murphy's law or whatever, but the very first time I tried to make changes I got the following exception at run time: Sys.WebForms.PageRequestManagerParserErrorException. After a little bit of Googling I found out the cause. I was using the UpdatePanel and using a Response.Write in my PageLoad. Eilon Lipton's blog post has more details.