Interesting Sessioin timeout issue

Q:Clients frequently got timeout in an uncertainty point.

A:Apart from asp.net Session.Timeout and the settingstate mark in the web.config, inproper IIS settings will result in trigger timeout as well
helpful details:
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In order to keep session state based on a stateless protocol - HTTP, web application keep the session detail in a process called w3wp.exe, if any events trigger w3wp.exe recyle then the session state stored in the proccess will be lost as well