After an upgrade from 1.318 to 1.321, I get messages like these on the console
every 10 seconds:

Aug 25, 2009 2:06:32 PM hudson.triggers.SafeTimerTask run
SEVERE: Timer task hudson.model.LoadStatistics$LoadStatisticsUpdater@8905bd failed
java.lang.NoClassDefFoundError: Could not initialize class hudson.model.ComputerSet
at hudson.model.Hudson.getComputer(Hudson.java:1253)
at hudson.model.LoadStatistics$LoadStatisticsUpdater.doRun(LoadStatistics.java:204)
at hudson.triggers.SafeTimerTask.run(SafeTimerTask.java:54)
at java.util.TimerThread.mainLoop(Timer.java:512)
at java.util.TimerThread.run(Timer.java:462)
Aug 25, 2009 2:06:35 PM hudson.triggers.SafeTimerTask run
SEVERE: Timer task hudson.slaves.NodeProvisioner$NodeProvisionerInvoker@1b595f3
failed
java.lang.NoClassDefFoundError: Could not initialize class hudson.model.ComputerSet
at hudson.model.Hudson.getComputer(Hudson.java:1253)
at
hudson.model.OverallLoadStatistics.computeIdleExecutors(OverallLoadStatistics.java:54)
at hudson.slaves.NodeProvisioner.update(NodeProvisioner.java:157)
at hudson.slaves.NodeProvisioner.access$000(NodeProvisioner.java:52)
at
hudson.slaves.NodeProvisioner$NodeProvisionerInvoker.doRun(NodeProvisioner.java:213)
at hudson.triggers.SafeTimerTask.run(SafeTimerTask.java:54)
at java.util.TimerThread.mainLoop(Timer.java:512)
at java.util.TimerThread.run(Timer.java:462)