http://mercurial.selenic.com/bts/issue619 $ hg init $ echo a > a $ hg ci -Ama adding a $ echo b > b $ hg branch b marked working directory as branch b $ hg ci -Amb adding b $ hg co -C 0 0 files updated, 0 files merged, 1 files removed, 0 files unresolved Fast-forward: $ hg merge b 1 files updated, 0 files merged, 0 files removed, 0 files unresolved (branch merge, don't forget to commit) $ hg ci -Ammerge Bogus fast-forward should fail: $ hg merge b abort: merging with a working directory ancestor has no effect [255]