Specifying grails.plugin.excludes (or plugin.excludes) in conf/Config.groovy with a Collection value e.g.:
grails.plugin.excludes = ['hibernate']
causes NullPointerException in org.codehaus.groovy.grails.plugins.PluginFilterRetriever in java.util.HashSet line 51; on that line erroneously passes includes, which will be null at that point, to the constructor of java.util.HashSet.
This was introduced back in march of 2010, so I'm unsure what other versions are affected, but 1.3.4 is.
Attached you'll find the patch for this.