When two ant tasks are run in parallel, sometimes the EXECUTOR_NUMBER for the
two tasks are the same. The EXECTOR_NUMBER variable should be unique for each
task. Here is the documentation URL for the EXECUTOR_NUMBER environment variable.

http://wiki.jenkins-ci.org/display/JENKINS/Building+a+software+project

Below are the steps to reproduce this issue.

Note: This defect seems to be non-deterministic. I will list the steps that,
when followed, will often produce the described defect.

1. Create four new Hudson jobs.
2. Configure each job. Under each job's "Build" section Add a build step,
select "invoke Ant".
3. Click the "Advanced" button under the "Invoke Ant" section. Paste the
following into the "Properties" text box.

EXECUTOR_NUMBER=$EXECUTOR_NUMBER

4. Configure the Ant build step of each job to run an ant task that prints out
the ${EXECUTOR_NUMBER} variable.
5. Setup Hudson to be able to run four executors simultaneously.
6. Launch each of the four jobs in parallel. Try to launch them at the same
time if possible.
7. Look at the console output of each of the jobs. On some runs, two separate
jobs will use the same executor number.
8. When step 7 has finished, if the defect has not manifested itself, repeat
steps 6 and 7.