1 - Create a grails project
2 - run-app or test-app your app
3 - add jars to the lib directory
4 - create a unit test or controller that uses the jar file
5 - run-app or test-app will give you NoClassDefFoundError errors
A work around is to do a grails clean after changing the contents of the lib directory. I'm assuming new jars should be automatically found without doing a clean.