With a job configuration like:

parent_job:
child_job
child_job

occasionally we'll get output that looks like this:

Starting build job child_job.
Starting build job child_job.
Finished Build : #123 - Job : child_job with status : SUCCESS
<this hangs forever, so we abort the parent_job>
Aborting all subjobs.
Finished Build : #124 - Job : child_job with status : ABORTED

However, job #124 will always have finished successfully. We see this relatively rarely (~5% of parent_job builds?). My guess is that there's a race condition causing this, since we usually see this during times of high load (ie many child_job instances are being started).