- Gaps in new issue numbers (for example ABC-5 then ABC-9)
- Users create a JIRA issue via email but in JIRA it doesn't exist
- Users create a JIRA issue via email but in JIRA it is the wrong issue
More than one JIRA instance is running, with the same set of projects, same base URL, and with the same email services listening to the same POP3 accounts. One of the instances is actually running at the base URL. One is running elsewhere.
Generally this happens with a development or test instance, created by a data export.