It was bug which got fixed in JBossAS-5.0.1 GA. I would recommend that you upgrade to the latest 5.1.0 GA version of JBoss AS since it has many bug fixes. Here are the details about this specific issue:
Please ShowSomeEffort. Saying "me too" on someone else's post is usually is not sufficient. You should open a new post and TellTheDetails. For example, are you deploying an EAR? Are you using JBoss AS 5.0.0.GA? What are the contents of your EAR?
Explaination: Normally all these kind of problems are cause dues to multiple version of same packages.
For example, In my eclipse environment, I have jsp-api and servlet-api of some version (lower then 2.5 and 2.0, respectively).
At runtime jboss compiles the jsp page and tries to invoke methods from newer version (Don't ask me which, since I have not dug in that line).
Solution: Remove servlet-api.jar and jsp-api.jar
TIP Whenever I run into these kind of issues, I rip apart the project by striping off all the jars. Then add them (in groups like spring, velocity, commons, etc.) one by one based on Class not found exception. Once my project runs, I know the offenders and I blacklist them by putting them in exclude tag in ivy or removing them from web-inf/lib, etc.
Its worth the time because its something I don't do everyday and keeps my project.
Yes, this runtime exception is absolutely due to conflicts in jar files.
I had servlet jar in my WEB-INF lib directory, which is added to build path to let the class visible to compile the dependecy files and another in my JBoss [Trinity] 4.2.1.GA server lib directory. When I ran the ear the application was running fine but when I ran the application on the my production server JBoss_5_1_0_GA it beautifully kissed that exception.
There can be two possibilties of error,
1) The jboss
2) The differences version of servlet jar files in EAR file and one in server directory.