Some text editors (i.e. emacs) leave a backup file when you save changes, with filename being the same as the original but ending with a ~. When using for example a /etc/portage/package.unmask directory, portage takes into account all files in that directory for unmasking... including files ending with ~. This means if you later remove an entry from one of these files, you either need to save twice or delete the backup - both an inconvenience and defeating the purpose of the backup in the first place.
This is fixed in svn r5548.
This has been released in 2.1.2_rc4-r9.
