404 error handler page executed even though no page not found error.

hello, I have created a web application in vs2008 ,c sharp (framework 3.5).I configured "web.config" to pass control to error1.aspx page in case of 404 error.Everytime I start to debug(& execute) project,error page get automatically executed even though there is no "page not found" error.Pls tell me solution for this problem.
Thanks in advance.