unit test failures: job.xml already exists

In my last 30 test runs on REL4 and Windows, I've seen this exception 6 times on Windows and 2 times on REL4 in the TestMiniMRDFSCaching or TestMiniMRClasspath logs:

[junit] java.io.IOException: Target C:/trunk/build/test/mapred/local/50064_1/taskTracker/jobcache/job_0001/job.xml already exists
[junit] at org.apache.hadoop.fs.FileUtil.checkDest(FileUtil.java:245)
[junit] at org.apache.hadoop.fs.FileUtil.copy(FileUtil.java:80)
[junit] at org.apache.hadoop.fs.LocalFileSystem.copyToLocalFile(LocalFileSystem.java:349)
[junit] at org.apache.hadoop.fs.FileSystem.copyToLocalFile(FileSystem.java:842)
[junit] at org.apache.hadoop.mapred.TaskTracker.localizeJob(TaskTracker.java:330)
[junit] at org.apache.hadoop.mapred.TaskTracker.startNewTask(TaskTracker.java:823)
[junit] at org.apache.hadoop.mapred.TaskTracker.offerService(TaskTracker.java:510)
[junit] at org.apache.hadoop.mapred.TaskTracker.run(TaskTracker.java:852)
[junit] at org.apache.hadoop.mapred.MiniMRCluster$TaskTrackerRunner.run(MiniMRCluster.java:141)
[junit] at java.lang.Thread.run(Thread.java:595)

In additon to the above exception, TestMiniMRClasspath also logged this exception further down in its log:

[junit] 2007-01-05 12:23:24,027 INFO mapred.JobInProgress (JobInProgress.java:completedTask(531)) - Job job_0001 has completed successfully.
[junit] 2007-01-05 12:23:24,058 INFO mapred.TaskTracker (TaskTracker.java:purgeJob(638)) - Received 'KillJobAction' for job: job_0001
[junit] 2007-01-05 12:23:24,058 INFO mapred.TaskTracker (TaskTracker.java:cleanup(1167)) - Error cleaning up task runner: java.lang.NullPointerException
[junit] at org.apache.hadoop.mapred.TaskTracker$TaskInProgress.cleanup(TaskTracker.java:1161)
[junit] at org.apache.hadoop.mapred.TaskTracker$TaskInProgress.jobHasFinished(TaskTracker.java:1079)
[junit] at org.apache.hadoop.mapred.TaskTracker.purgeJob(TaskTracker.java:650)
[junit] at org.apache.hadoop.mapred.TaskTracker.access$100(TaskTracker.java:51)
[junit] at org.apache.hadoop.mapred.TaskTracker$1.run(TaskTracker.java:151)
[junit] at java.lang.Thread.run(Thread.java:595)