In the ASF Jenkins, we do not allow builds to run on master. Thus, we have set the number of executors on master to zero. Even so, one build recently (https://builds.apache.org/hudson/job/Axis2/774/) got assigned to run on master. The build is configured with a label to run on one of the slaves, but somehow Jenkins assigned it incorrectly.
Running on master also meant it got stuck, presumably due to the lack of executors, without any way of stopping it.