1.479-SNAPSHOT. Set system quiet period to two seconds. Click Build Now next to a job, and when the queue item appears, click the red X next to it to cancel. Sometimes you will be shown http://localhost:8080/queue/item/21/cancelQueue as an error page:
Presumably item #21 had already been sent to an executor, but the AJAX code to refresh the queue & executor list had not yet gotten a chance to run. Fine, but Jenkins should recover more gracefully than this - just redirect to the calling page and ignore the request.