Split as new bug from https://issues.jenkins-ci.org/browse/JENKINS-16979

Also reported here: http://stackoverflow.com/questions/15268736/error-publisher-org-jenkinsci-plugins-discardbuild-discardbuildpublisher-aborte

If a post-build action to 'Discard Old Builds' is added, this always results:

Archiving artifacts
Discard old builds...
ERROR: Publisher org.jenkinsci.plugins.discardbuild.DiscardBuildPublisher aborted due to exception
java.lang.UnsupportedOperationException
at java.util.AbstractList.add(AbstractList.java:148)
at java.util.AbstractList.add(AbstractList.java:108)
at org.jenkinsci.plugins.discardbuild.DiscardBuildPublisher.perform(DiscardBuildPublisher.java:148)
at hudson.tasks.BuildStepMonitor$3.perform(BuildStepMonitor.java:36)
at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:798)
at hudson.model.AbstractBuild$AbstractBuildExecution.performAllBuildSteps(AbstractBuild.java:770)
at hudson.model.Build$BuildExecution.post2(Build.java:183)
at hudson.model.AbstractBuild$AbstractBuildExecution.post(AbstractBuild.java:720)
at hudson.model.Run.execute(Run.java:1600)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:237)

and the build fails.