I just had this problem, here are the steps that brought this error message. I will try to reproduce later with a public svn repository. Here are the logs to help me reproduce it later.

Note that we use subversion in a braindead manner (i.e. we commit inside the tags).

Subversion side :
We have tagged twice our trunk, first with tag "r3", then with tag "r4". Then, we did several commits in both tags, both from svn and from bzr (by pulling changes then committing).

Bazaar side:
1) Branch from the subversion tag r3
repo$ bzr branch svn://host/repo/tags/r3
2) Branch from the subversion tag r4
repo$ bzr branch svn://host/repo/tags/r4
3) Try to merge or pull a revision from the bzr branch "r3" into the branch "r4"
repo/r4$ bzr merge -r1218 ../r3/

=> Crash with the following message

bzr: ERROR: bzrlib.errors.BzrCheckError: Internal check failed: Cannot add revision(s) to repository: missing chk node(s) for id_to_entry maps

Traceback (most recent call last):
  File "bzrlib\commands.pyo", line 853, in exception_to_return_code
  File "bzrlib\commands.pyo", line 1055, in run_bzr
  File "bzrlib\commands.pyo", line 661, in run_argv_aliases
  File "bzrlib\commands.pyo", line 665, in run_direct
  File "bzrlib\cleanup.pyo", line 122, in run_simple
  File "bzrlib\cleanup.pyo", line 156, in _do_with_cleanups
  File "bzrlib\builtins.pyo", line 3794, in run
  File "bzrlib\builtins.pyo", line 3923, in _get_merger_from_branch
  File "bzrlib\merge.pyo", line 363, in from_revision_ids
  File "bzrlib\merge.pyo", line 505, in set_other_revision
  File "bzrlib\merge.pyo", line 522, in _maybe_fetch
  File "bzrlib\decorators.pyo", line 194, in write_locked
  File "bzrlib\branch.pyo", line 593, in fetch
  File "bzrlib\repository.pyo", line 1704, in fetch
  File "bzrlib\decorators.pyo", line 194, in write_locked
  File "bzrlib\repository.pyo", line 3432, in fetch
  File "bzrlib\fetch.pyo", line 83, in __init__
  File "bzrlib\fetch.pyo", line 109, in __fetch
  File "bzrlib\fetch.pyo", line 137, in _fetch_everything_for_search
  File "bzrlib\repository.pyo", line 4274, in insert_stream
  File "bzrlib\repository.pyo", line 4377, in _locked_insert_stream
  File "bzrlib\repository.pyo", line 1563, in commit_write_group
  File "bzrlib\repofmt\pack_repo.pyo", line 2314, in _commit_write_group
  File "bzrlib\repofmt\pack_repo.pyo", line 2152, in _commit_write_group
BzrCheckError: Internal check failed: Cannot add revision(s) to repository: missing chk node(s) for id_to_entry maps

bzr 2.1.0 on python 2.5.4 (Windows-XP-5.1.2600-SP2)
arguments: ['c:\\Program Files\\Bazaar\\bzr.exe', 'merge', '-r1218', '../2.42r3/']
encoding: 'cp1252', fsenc: 'mbcs', lang: None
plugins:
  bzrtools             C:\Program Files\Bazaar\plugins\bzrtools [2.1.0]
  explorer             C:\Program Files\Bazaar\plugins\explorer [1.0.0rc1]
  launchpad            C:\Program Files\Bazaar\plugins\launchpad [2.1.0]
  netrc_credential_store C:\Program Files\Bazaar\plugins\netrc_credential_store [2.1.0]
  qbzr                 C:\Program Files\Bazaar\plugins\qbzr [0.18.1]
  rebase               C:\Program Files\Bazaar\plugins\rebase [0.5.5]
  svn                  C:\Program Files\Bazaar\plugins\svn [1.0.2]
  upload               C:\Program Files\Bazaar\plugins\upload [1.0.0dev]
  xmloutput            C:\Program Files\Bazaar\plugins\xmloutput [0.8.6]

*** Bazaar has encountered an internal error.  This probably indicates a
    bug in Bazaar.  You can help us fix it by filing a bug report at
        https://bugs.launchpad.net/bzr/+filebug
    including this traceback and a description of the problem.



Note that we will keep the subversion repository intact, and this can be reproducted here with two branches and a merge attempt.