Immediately after a promotion, the slave executing the promotion crashed and did not recover. The explanation on the help page say submit an issue, so here it is. The Jenkins log on the crashed slave shows no issues. The master log shows:

Jul 28, 2015 9:29:31 AM INFO hudson.model.Run execute

dns-hijacker/promotion/manual #4 main build action completed: FAILURE

Jul 28, 2015 9:29:44 AM SEVERE hudson.model.Executor run

Unexpected executor death
java.lang.IllegalStateException: /var/lib/jenkins/jobs/dns-hijacker/promotions/manual/builds/4 already existed; will not overwite with dns-hijacker/promotion/manual #4
at hudson.model.RunMap.put(RunMap.java:189)
at jenkins.model.lazy.LazyBuildMixIn.newBuild(LazyBuildMixIn.java:178)
at hudson.model.AbstractProject.newBuild(AbstractProject.java:1017)
at hudson.model.AbstractProject.createExecutable(AbstractProject.java:1216)
at hudson.model.AbstractProject.createExecutable(AbstractProject.java:144)
at hudson.model.Executor$1.call(Executor.java:335)
at hudson.model.Executor$1.call(Executor.java:317)
at hudson.model.Queue._withLock(Queue.java:1345)
at hudson.model.Queue.withLock(Queue.java:1210)
at hudson.model.Executor.run(Executor.java:317)