Not very sensible error message coming out:
/Users/marcpalmer/Projects/grails/grails/ant/build/unit-test.xml:72: org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed, /Users/marcpalmer/Projects/grails/grails/test/groovy/org/codehaus/groovy/grails/plugins/web/ControllersGrailsPluginTests.groovy: -1: reference to AbstractGrailsPluginTests is ambigous, both class org.codehaus.groovy.grails.plugins.web.AbstractGrailsPluginTests and org.codehaus.groovy.grails.web.plugins.AbstractGrailsPluginTests match
@ line -1, column -1.