Specifically,
- if BUG-123 branch was placed in packed-refs file in the past
(which may be older than what you have right now at heads/BUG-123
as a loose ref), the above procedure will still make it appear in
your "git branch --list" output, pointing at a possibly old
commit that may even have been pruned away.
- if BUG-123 branch was placed in packed-refs file and you haven't
touched that branch since then, heads/BUG-123 file would not
exist, "mv" will fail, and you won't see closed/BUG-123 at the
end of the procedure.