I had an issue that was accidentally assigned to the main Jira Admin user. From what I can tell from the emails I received, it caused an email to be sent to the admin user notifying them that they were assigned. The admin user must have automatically replied with a comment along the lines of

JIRA Admin commented on ACIS-5816:
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[ http://jira.coemergence.com/jira//browse/ACIS-5816?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_59779 ]

This caused an endless loop of email, comment, email, comment. It eventually locked JIRA. I was able to fix it by ultimately deleting the issue directly from the DB.