After upgrading the mercurial plugin from 1.37 to 1.38, I get the following messages in the logs:

Dec 3, 2011 3:46:27 PM hudson.util.RobustReflectionConverter doUnmarshal
WARNING: Skipping a non-existent field forest
com.thoughtworks.xstream.converters.reflection.NonExistentFieldException: No such field hudson.plugins.mercurial.MercurialSCM.forest
at com.thoughtworks.xstream.converters.reflection.FieldDictionary.field(FieldDictionary.java:106)
at com.thoughtworks.xstream.converters.reflection.PureJavaReflectionProvider.getFieldType(PureJavaReflectionProvider.java:152)
at hudson.util.RobustReflectionConverter.determineType(RobustReflectionConverter.java:356)
at hudson.util.RobustReflectionConverter.doUnmarshal(RobustReflectionConverter.java:229)
at hudson.util.RobustReflectionConverter.unmarshal(RobustReflectionConverter.java:180)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convert(TreeUnmarshaller.java:82)
at com.thoughtworks.xstream.core.AbstractReferenceUnmarshaller.convert(AbstractReferenceUnmarshaller.java:63)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:76)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:60)
at hudson.util.RobustReflectionConverter.unmarshalField(RobustReflectionConverter.java:290)
at hudson.util.RobustReflectionConverter.doUnmarshal(RobustReflectionConverter.java:233)
at hudson.util.RobustReflectionConverter.unmarshal(RobustReflectionConverter.java:180)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convert(TreeUnmarshaller.java:82)
at com.thoughtworks.xstream.core.AbstractReferenceUnmarshaller.convert(AbstractReferenceUnmarshaller.java:63)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:76)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:60)
at com.thoughtworks.xstream.core.TreeUnmarshaller.start(TreeUnmarshaller.java:137)
at com.thoughtworks.xstream.core.AbstractTreeMarshallingStrategy.unmarshal(AbstractTreeMarshallingStrategy.java:33)
at com.thoughtworks.xstream.XStream.unmarshal(XStream.java:926)
at hudson.util.XStream2.unmarshal(XStream2.java:91)
at com.thoughtworks.xstream.XStream.unmarshal(XStream.java:912)
at com.thoughtworks.xstream.XStream.fromXML(XStream.java:864)
at hudson.XmlFile.read(XmlFile.java:133)
at hudson.model.Items.load(Items.java:114)
at jenkins.model.Jenkins$14.run(Jenkins.java:2357)
at org.jvnet.hudson.reactor.TaskGraphBuilder$TaskImpl.run(TaskGraphBuilder.java:146)
at org.jvnet.hudson.reactor.Reactor.runTask(Reactor.java:259)
at jenkins.model.Jenkins$5.runTask(Jenkins.java:798)
at org.jvnet.hudson.reactor.Reactor$2.run(Reactor.java:187)
at org.jvnet.hudson.reactor.Reactor$Node.run(Reactor.java:94)
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:680)