Described here.

Debug logging looks like this:
2008-10-14 15:33:43,646 DEBUG [main] [atlassian.plugin.classloader.PluginsClassLoader] findClass Find class [ com.adaptavist.confluence.backups.UpdateHousekeepingAction ], found [ class com.adaptavist.confluence.backups.UpdateHousekeepingAction ]
2008-10-14 15:33:43,646 DEBUG [main] [atlassian.plugin.classloader.PluginsClassLoader] findClass Find class [ com.adaptavist.confluence.backups.UpdateHousekeepingAction ], found [ class com.adaptavist.confluence.backups.UpdateHousekeepingAction ]
2008-10-14 15:33:43,678 ERROR [main] [ContainerBase.[Standalone].[localhost].[/]] listenerStart Exception sending context initialized event to listener instance of class com.atlassian.upgrade.UpgradeLauncherServletContextListener
java.lang.NoClassDefFoundError: com/atlassian/confluence/core/actions/NotFoundAction

We should check for this class and disable the plugin before the plugin and class loads.