When comments are created by email and notifications result in another comment (usually involving a second system similarly configured) an unbounded mail loop is created.

Today SAC was bought down by two JIRAs cross connected via their mailboxes.

Someone created an account for enterprise-hosted-support@atlassian.com on their JIRA, this caused SAC to create an issue on the EHSP support queue, and reply back to the reporter with an email saying it had done so. This caused the remote JIRA to create an issue on it's side and send a notification that it had done so back to SAC. Because this email contained the EHSP key, SAC filed the email as a comment, and replied telling it had done so. This caused the remote to create a new issue (as the issue key didn't match on its side), and reply to SAC ...

hilarity did not ensue

Also note the Ubisoft + JAC loop that happened here: http://jira.atlassian.com/browse/JRA-5865