I have a job that is devoted to make a release with Maven (for various reasons,
I'm not using the specific Hudson support, but driving Maven explicitly).
Unfortunately I've just discovered that it made a wrong release, since after
pulling the changes from Mercurial, it didn't do an update:

Started by user fabrizio.giudici
Building remotely on Hetzner
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
pulling from hg.bundle
searching for changes
adding changesets
adding manifests
adding file changes
added 3 changesets with 18 changes to 18 files (+1 heads)
not updating, since new heads added
(run 'hg heads' to see heads, 'hg merge' to merge)
Parsing POMs

Not that "not updating since new heads added". A rather old blog
(http://benjamin.smedbergs.us/blog/2008-10-09/release-branches-in-mozilla-central/)
describes it as a bug when running hg pull -u - I suppose it hasn't been fixed
yet (hg 1.3.1).

The result is that Hudson performed a release with the old stuff. To fix it,
Hudson needs to perform an explicit hg update -C after the pull. Please consider
this with a high priority, since it's blocking a release workflow.