After activating the JIRA Greenhopper plugin, and viewing the /secure/GreenHopper.jspa?decorator=admin admin link, I get a stacktrace:

HTTP Status 404 - Could not execute action [GreenHopper]:Action 'GreenHopper' not found or could not be initialized: Class not found<p><small><small><pre>java.lang.IllegalArgumentException: Action 'GreenHopper' not found or could not be initialized: Class not found at com.atlassian.jira.config.webwork.JiraActionFactory$JiraPluginActionFactory.getActionImpl(JiraActionFactory.java:60) at webwork.action.factory.ScriptActionFactoryProxy.getActionImpl(ScriptActionFactoryProxy.java:54) at webwork.action.factory.XMLActionFactoryProxy.getActionImpl(XMLActionFactoryProxy.java:54) at
...

A JIRA restart is required to clear this up.