At "completion" of job, when post-build steps are invoked builds can hang waiting for a checkpoint on a separate build that isn't related to the current job's build. The message that is displayed is "Stash Pull Request Builder - Post Build Comment is waiting for a checkpoint on <PROJECT_NAME> <BUILD_NUMBER>." It appears that there are several other plugins that have ran into this problem in the past--the fix appears to be well documented. Easy to find other reported issues by searching https://issues.jenkins-ci.org for "serialize." This ultimately ends up clogging up job queues, and artificially holding executor positions open that could have otherwise completed/released.