I was trying to add the SCM Sync configuration to my Jenkins installation. It fails with the following trace.
The odd thing is that the file in question has only just been created (as part of the installation).
It is repeatable: I stopped the service, deleted the entire plugin dir and restarted, retried: same fault.

24-Oct-2012 11:35:03 hudson.model.UpdateCenter$DownloadJob run
SEVERE: Failed to install SCM Sync configuration plugin
hudson.util.IOException2: Failed to dynamically deploy this plugin
at hudson.model.UpdateCenter$InstallationJob._run(UpdateCenter.java:1227)
at hudson.model.UpdateCenter$DownloadJob.run(UpdateCenter.java:1037)
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
at java.util.concurrent.FutureTask$Sync.innerRun(Unknown Source)
at java.util.concurrent.FutureTask.run(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
Caused by: java.io.IOException: Unable to delete C:\Jenkins\plugins\scm-sync-configuration\WEB-INF\lib\antlr-2.7.6.jar
at hudson.Util.deleteFile(Util.java:243)
at hudson.Util.deleteRecursive(Util.java:293)
at hudson.Util.deleteContentsRecursive(Util.java:204)
at hudson.Util.deleteRecursive(Util.java:284)
at hudson.Util.deleteContentsRecursive(Util.java:204)
at hudson.Util.deleteRecursive(Util.java:284)
at hudson.Util.deleteContentsRecursive(Util.java:204)
at hudson.Util.deleteRecursive(Util.java:284)
at hudson.ClassicPluginStrategy.explode(ClassicPluginStrategy.java:403)
at hudson.ClassicPluginStrategy.createPluginWrapper(ClassicPluginStrategy.java:117)
at hudson.PluginManager.dynamicLoad(PluginManager.java:388)
at hudson.model.UpdateCenter$InstallationJob._run(UpdateCenter.java:1223)
... 7 more
24-Oct-2012 11:35:02 hudson.PluginManager dynamicLoad
INFO: Attempting to dynamic load C:\Jenkins\plugins\scm-sync-configuration.jpi
24-Oct-2012 11:35:02 hudson.model.UpdateCenter$UpdateCenterConfiguration download
INFO: Downloading SCM Sync configuration plugin
24-Oct-2012 11:35:01 hudson.model.UpdateCenter$DownloadJob run
INFO: Starting the installation of SCM Sync configuration plugin on behalf of anonymous
24-Oct-2012 11:34:48 hudson.WebAppMain$2 run
INFO: Jenkins is fully up and running