Given a repo that has multiple branches, "default" and "coolbranch".
Clone the repo, notice that current branch is "default".  Do a pull, do
an update just to make sure you're all updated.  Make some change to
the working copy, commit it, and push.

pushing to /home/jkeating/test/head
searching for changes
abort: push creates new remote branches!
(did you forget to merge? use push -f to force)

Response from mailing list was:

This is a bug in hg, the network stuff isn't really aware of named
branches, it will only notice that the number of heads changes.