I have my mercurial build setup on a ubuntu slave, and get the following in polling log if the number of executors on the master is set to 0.

ERROR: Failed to record SCM polling
java.lang.NullPointerException
at hudson.model.Run.getEnvironment(Run.java:1648)
at hudson.model.AbstractBuild.getEnvironment(AbstractBuild.java:663)
at hudson.plugins.mercurial.MercurialSCM.compareRemoteRevisionWith(MercurialSCM.java:250)
at hudson.scm.SCM._compareRemoteRevisionWith(SCM.java:351)
at hudson.scm.SCM.poll(SCM.java:368)
at hudson.model.AbstractProject.poll(AbstractProject.java:1153)
at hudson.triggers.SCMTrigger$Runner.runPolling(SCMTrigger.java:330)
at hudson.triggers.SCMTrigger$Runner.run(SCMTrigger.java:359)
at hudson.util.SequentialExecutionQueue$QueueEntry.run(SequentialExecutionQueue.java:118)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:441)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
at java.util.concurrent.FutureTask.run(FutureTask.java:138)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:619)