This bug may occur in versions 2.5 and below. It occurs when you add at least two mappings of the same type, delete the first mapping and then attempt to add another one.
The ideal solution is to upgrade to the latest version where this has been fixed.
Failing that a workaround is to delete all similar mappings e.g. all constituent codes, gift attributes or, in the above case, all appeal mappings and add them again.