In general this kind of exception might come.
[LIST] If session is not removed after its usuage.
[LIST] If the session timeout is more and you are using
alot of session objects without cleaning the unwanted sessions.
[LIST] Do avoid usage of session at the max. as it would slow the process of your application.
[LIST] Dont store large number of data in session, if its stored do remove it after its usage.
Hope this works out.
regards
Magesh`