This also occurs for the Aeryn Subversion repository at svn+http://aeryn.tigris.org/svn/aeryn/trunk, same exception but with a tag of tags/aeryn_2_1_2.  I removed all the tags that were present:

|> bzr tags
README               ?
aeryn_2_1_2          ?
aeryn_2_1_2_gefanuc_06_11_2007 ?
511 balin:~/Repositories/Bazaar/Subversion_Branches/Aeryn_Trunk
|> for  t in README aeryn_2_1_2 aeryn_2_1_2_gefanuc_06_11_2007 ; do bzr tag --delete $t ; done
Deleted tag README.
Deleted tag aeryn_2_1_2.
Deleted tag aeryn_2_1_2_gefanuc_06_11_2007.
|> bzr tags
|>

but the error still occurs:

|> bzr pull
Using saved location: svn+http://aeryn.tigris.org/svn/aeryn/trunk
using experimental bzr-svn mappings; output may change between revisions
bzr: ERROR: exceptions.KeyError: 'tags/aeryn_2_1_2'                                                                                            

Traceback (most recent call last):
  File "/home/Checkouts/Bazaar/Bazaar_Mainline/bzrlib/commands.py", line 857, in run_bzr_catch_errors
    return run_bzr(argv)
  File "/home/Checkouts/Bazaar/Bazaar_Mainline/bzrlib/commands.py", line 797, in run_bzr
    ret = run(*run_argv)
  File "/home/Checkouts/Bazaar/Bazaar_Mainline/bzrlib/commands.py", line 499, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/home/Checkouts/Bazaar/Bazaar_Mainline/bzrlib/builtins.py", line 665, in run
    possible_transports=possible_transports)
  File "/home/Checkouts/Bazaar/Bazaar_Mainline/bzrlib/decorators.py", line 192, in write_locked
    result = unbound(self, *args, **kwargs)
  File "/home/Checkouts/Bazaar/Bazaar_Mainline/bzrlib/workingtree.py", line 1521, in pull
    possible_transports=possible_transports)
  File "/home/Checkouts/Bazaar/Bazaar_Mainline/bzrlib/decorators.py", line 192, in write_locked
    result = unbound(self, *args, **kwargs)
  File "/home/Checkouts/Bazaar/Bazaar_Mainline/bzrlib/branch.py", line 1853, in pull
    _override_hook_target=_override_hook_target)
  File "/home/Checkouts/Bazaar/Bazaar_Mainline/bzrlib/decorators.py", line 192, in write_locked
    result = unbound(self, *args, **kwargs)
  File "/home/Checkouts/Bazaar/Bazaar_Mainline/bzrlib/branch.py", line 1655, in pull
    result.tag_conflicts = source.tags.merge_to(self.tags, overwrite)
  File "/home/Checkouts/Bazaar/Bazaar_Mainline/bzrlib/tag.py", line 207, in merge_to
    source_dict = self.get_tag_dict()
  File "/home/users/russel/.bazaar/plugins/svn/branch.py", line 100, in get_tag_dict
    layout=self.layout)
  File "/home/Checkouts/Bazaar/Bazaar_Mainline/bzrlib/decorators.py", line 138, in read_locked
    result = unbound(self, *args, **kwargs)
  File "/home/users/russel/.bazaar/plugins/svn/repository.py", line 742, in find_tags
    del paths[bp]
KeyError: 'tags/aeryn_2_1_2'

bzr 1.6b3 on python 2.5.2 (linux2)
arguments: ['/home/users/russel/bin/bzr', 'pull']
encoding: 'UTF-8', fsenc: 'UTF-8', lang: 'en_GB.UTF-8'
plugins:
  builddeb             /usr/lib/python2.5/site-packages/bzrlib/plugins/builddeb [0.92.0dev0]
  bzrtools             /home/users/russel/.bazaar/plugins/bzrtools [1.6.0]
  dbus                 /usr/lib/python2.5/site-packages/bzrlib/plugins/dbus [unknown]
  email                /usr/lib/python2.5/site-packages/bzrlib/plugins/email [unknown]
  gtk                  /home/users/russel/.bazaar/plugins/gtk [0.95.0dev1]
  launchpad            /home/Checkouts/Bazaar/Bazaar_Mainline/bzrlib/plugins/launchpad [unknown]
  pqm                  /usr/lib/python2.5/site-packages/bzrlib/plugins/pqm [1.0.0dev0]
  rebase               /home/users/russel/.bazaar/plugins/rebase [0.4.0dev0]
  svn                  /home/users/russel/.bazaar/plugins/svn [0.4.11exp0]
  xmloutput            /home/users/russel/.bazaar/plugins/xmloutput [0.4.4]
*** Bazaar has encountered an internal error.
    Please report a bug at https://bugs.launchpad.net/bzr/+filebug
    including this traceback, and a description of what you
    were doing when the error occurred.