I get the following error with an ant or maven build from workspace or repository.

Started by user anonymous
FATAL: null
java.lang.UnsupportedOperationException
at java.util.AbstractMap$SimpleImmutableEntry.setValue(Unknown Source)
at hudson.EnvVars.resolve(EnvVars.java:178)
at hudson.model.AbstractBuild.getEnvironment(AbstractBuild.java:708)
at hudson.plugins.mercurial.MercurialSCM.update(MercurialSCM.java:430)
at hudson.plugins.mercurial.MercurialSCM.checkout(MercurialSCM.java:419)
at hudson.model.AbstractProject.checkout(AbstractProject.java:1082)
at hudson.model.AbstractBuild$AbstractRunner.checkout(AbstractBuild.java:479)
at hudson.model.AbstractBuild$AbstractRunner.run(AbstractBuild.java:411)
at hudson.model.Run.run(Run.java:1273)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:137)

It seems if you use -XX:+AggressiveOpts then alt-rt.jar is loaded in the classpath earlier than rt.jar. This results in a different implementation of the Map collections.

Recreate by starting the server with jvm option -XX:+AggressiveOpts. See arguments.cpp and altclasses/java/util/TreeMap.java in in the jdk6 source tree.

Workaround by removing -XX:+AggressiveOpts.

I think this is the same bug reported in JENKINS-7159 and JENKINS-6055 .