Mozilla has been running into this. I still think that --force should not be
needed for (re-)starting a branch, but mpm disagrees.
Perhaps it is useful to point out that this happens on a daily basis in
our environment. It goes like this:
Developer A works on new-feature branch, decides to merge changes
from bugfix branch.
Developer B works on bugfix branch, commits a change and tries to
push.
Since we have some A-type developers who wish to merge often (perhaps
more than once per day), all of the B-type developers cannot foresee
whether the "-f" warning printed by Mercurial should be ignored or
heeded.