The plugin system does not catch errors unzipping plugins retrieved from the database. So if a corrupt plugin jar creeps in, that can break the entire plugin system. Example stack trace:

2008-08-08 00:04:09,481 INFO [main] [com.atlassian.confluence.lifecycle] contextInitialized Starting Confluence 2.9 (build #1415)
2008-08-08 00:04:48,466 ERROR [main] [atlassian.confluence.event.ConfluenceEventManager] publishEvent An exception was encountered while processing the event: com.atlassian.confluence.event.events.admin.ConfluenceReadyEvent[source=com.atlassian.confluence.upgrade.impl.DefaultUpgradeManager@663ec]
java.lang.IllegalStateException: error in opening zip file
at com.atlassian.plugin.classloader.PluginClassLoader.<init>(PluginClassLoader.java:51)
at com.atlassian.plugin.loaders.ClassLoadingPluginLoader.deployPluginFromUnit(ClassLoadingPluginLoader.java:89)
at com.atlassian.plugin.loaders.ClassLoadingPluginLoader.loadAllPlugins(ClassLoadingPluginLoader.java:64)
at com.atlassian.confluence.plugin.DatabaseClassLoadingPluginLoader.loadAllPlugins(DatabaseClassLoadingPluginLoader.java:67)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)

Caused by PLUG-113