I found this one a bit amusing.
One of our users included an email-in email address (xxxx@ourdiscoursesite.nz) in a reply to one of our site’s notification emails (which was Cc’d).
This resulted in a staged user being created with the sender’s name and the notifications@ourdiscoursesite.nz being the associated email address.
While this is a bit weird, there was no harm done.
This shouldn’t be possible I don’t think - perhaps the system emails should be excluded from creating staged users?