ClassCastException

I have been working on JSPs javabeans and EJBs One of the common errors received is ClassCastException when a JSP is trying to communicate with a javabean. To solve this problem I have to shut down my weblogic server and start it up again and this consumes a lot of time as I have about 70
ejbs deployed.

I would appreciate it if you could let me know under what circumstances does this error occur.