If you change the Host in a node's configuration, and save the configuration, it will immediately restart the node, killing any jobs that were running on that node. It would be nice if it would at least warn you that there are jobs running on this node, and that saving the configuration will result in the node restarting and the job being killed (in my case I was changing the host from IP address to hostname, so it was less obvious that this was going to be disruptive). Even better would be if Jenkins could offer to restart the node after the running jobs complete.