The "priority" field in Bugzilla used to be visible and sometimes it was set to something other than "normal". Now the field has been hidden, but when someone is added to the CC: list on a bug with a priority other than normal, it is automatically changed to normal priority.
This isn't really an issue if the priority field is obsolete, but it does send unnecessary e-mail notifications: Adding a CC: doesn't usually send e-mail notifications, but priority change does.
Version: unspecified
Severity: enhancement