For jobs that have concurrent builds enabled and use jira-plugin as a post-build step, jira-plugin with hang with this message if later builds complete before earlier builds:
Update relevant JIRA issues is waiting for a checkpoint on foo #123
This can lead to a lot of idle executors.
This behavior is related to getRequiredMonitorService() returning BuildStepMonitor.BUILD instead of BuildStepMonitor.NONE.
While searching for why this is happening, I see some plugins have resolved this by simply changing the value returned (https://github.com/jenkinsci/nunit-plugin/commit/1e89e0267814d280b7051d7d70d5d2939326b182) while other plugins needed slightly more complicated solutions (https://github.com/jglick/email-ext-plugin/commit/6fb41818d63ca0067171f8c01fcdbed196335b44)
Although I can make the 1 line change, I do not know plugin well enough to know if its that simple.