$ hg init

  $ echo "nothing" > a

  $ hg add a

  $ hg commit -m ancestor

  $ echo "something" > a

  $ hg commit -m branch1

  $ hg co 0

  1 files updated, 0 files merged, 0 files removed, 0 files unresolved

  $ echo "something else" > a

  $ hg commit -m branch2

  created new head

  $ hg merge 1

  merging a

  warning: conflicts during merge.

  merging a failed!

  0 files updated, 0 files merged, 0 files removed, 1 files unresolved

  use 'hg resolve' to retry unresolved file merges or 'hg update -C' to abandon

  $ hg id

  32e80765d7fe+75234512624c+ tip

  $ cat a

  <<<<<<< local

  something else

  =======

  something

  >>>>>>> other

  $ hg status

  M a

  ? a.orig