error upgrading bazaar to 1.16…I think.

Nothing is broken. Except for your 'rebase' plugin that hasn't been loaded because it isn't up to date with bzr anymore.
Upgrade it or get rid of it and all should be fine again.

I see no rebase plugin in ~/.bazaar/plugins/ …

I don't suppose plugins include other plugins beneath the /plugins/ directory?

Run 'bzr plugins -v' and it should tell you where it is.

Oddly enough, it's not listed

…

$ bzr plugins -v
Unable to load plugin 'rebase'. It requested API version (1, 13, 0) of module <module 'bzrlib' from '/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/bzrlib/__init__.pyc'> but the minimum exported version is (1, 15, 0), and the maximum is (1, 16, 0)
auth
    (no description)
    /Users/tony/.bazaar/plugins/auth

automv
    Automatically detect renames/moves in the working tree.
    /Users/tony/.bazaar/plugins/automv

bzrtools 1.16
    Various useful commands for working with bzr.
    /Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/bzrlib/plugins/bzrtools

cvsps
    A plugin for converting from CVS to bzr using a cvsps changeset listing.
    /Users/tony/.bazaar/plugins/cvsps

email
    Sending emails for commits and branch changes.
    /Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/bzrlib/plugins/email

extmerge
    external merge plugin for bzr
    /Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/bzrlib/plugins/extmerge

fastimport 0.8dev
    FastImport Plugin
    /Users/tony/.bazaar/plugins/fastimport

interactive 1.2
    User interaction support for Bazaar.
    /Users/tony/.bazaar/plugins/interactive

launchpad 1.16
    Launchpad.net integration plugin for Bazaar.
    /Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/bzrlib/plugins/launchpad

log_rss 0.1dev
    Add a log formatter that generates an RSS feed.
    /Users/tony/.bazaar/plugins/log_rss

loom 1.4dev
    Loom is a bzr plugin which adds new commands to manage a loom of patches.
    /Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/bzrlib/plugins/loom

netrc_credential_store 1.16
    Use ~/.netrc as a credential store for authentication.conf.
    /Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/bzrlib/plugins/netrc_credential_store

notification
    GUI Notification plugin for bzr
    /Users/tony/.bazaar/plugins/notification

plugin_info 1.3dev
    plugin-info is a bzr plugin for working with the bzr plugins.
    /Users/tony/.bazaar/plugins/plugin_info

push_and_update 0.2dev
    A plugin for pushing and updating a remote repository.
    /Users/tony/.bazaar/plugins/push_and_update

qbzr 0.9.9
    QBzr - Qt-based frontend for Bazaar
    /Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/bzrlib/plugins/qbzr

search 1.7dev
    search is a bzr plugin for searching bzr content.
    /Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/bzrlib/plugins/search

stats
    A Simple bzr plugin to generate statistics about the history.
    /Users/tony/.bazaar/plugins/stats

svn 0.5.3
    Support for Subversion branches
    /Users/tony/.bazaar/plugins/svn

upload 1.0dev
    Upload a working tree, incrementally.
    /Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/bzrlib/plugins/upload

usertest
    Plugin to run user scripts on custom source trees.
    /Users/tony/.bazaar/plugins/usertest

xmloutput 0.8.4
    This plugin provides xml output for status, log, annotate, missing, info,
    /Users/tony/.bazaar/plugins/xmloutput

"bzr plugins" might not list plugins that couldn't be loaded.

It's a pain, but you can search for the traceback in .bzr.log (use "bzr
version" to find its location). It should show bzr-rebase's path.

Okay, so the installer for Bazaar 1.16 for OS X was finally released, so I installed it.

Afterwards, 'bzr selftest' gave me the following:

***

ony$ bzr selftest
Unable to load plugin 'rebase'. It requested API version (1, 13, 0) of module <module 'bzrlib' from '/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/bzrlib/__init__.pyc'> but the minimum exported version is (1, 15, 0), and the maximum is (1, 16, 0)
/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/Crypto/Hash/SHA.py:6: DeprecationWarning: the sha module is deprecated; use the hashlib module instead
  from sha import *
/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/Crypto/Hash/MD5.py:6: DeprecationWarning: the md5 module is deprecated; use hashlib instead
  from md5 import *
bzr: ERROR: exceptions.AssertionError: transport module bzrlib.plugins.auth.auth_remote doesn't provide get_test_permutations()

Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/bzrlib/commands.py", line 729, in exception_to_return_code
    return the_callable(*args, **kwargs)
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/bzrlib/commands.py", line 924, in run_bzr
    ret = run(*run_argv)
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/bzrlib/commands.py", line 560, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/bzrlib/builtins.py", line 3359, in run
    result = selftest(**selftest_kwargs)
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/bzrlib/tests/__init__.py", line 3146, in selftest
    suite = test_suite(keep_only, starting_with)
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/bzrlib/tests/__init__.py", line 3542, in test_suite
    suite.addTest(loader.loadTestsFromModuleNames(testmod_names))
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/bzrlib/tests/TestUtil.py", line 92, in loadTestsFromModuleNames
    result.addTests(self.loadTestsFromModuleName(name))
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/bzrlib/tests/TestUtil.py", line 99, in loadTestsFromModuleName
    result.addTests(self.loadTestsFromModule(module))
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/bzrlib/tests/TestUtil.py", line 129, in loadTestsFromModule
    return load_tests(basic_tests, module, self)
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/bzrlib/tests/test_read_bundle.py", line 39, in load_tests
    return tests.multiply_tests(transport_tests, transport_test_permutations(),
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/bzrlib/tests/test_transport_implementations.py", line 80, in transport_test_permutations
    reduce(getattr, (module).split('.')[1:], __import__(module)))
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/bzrlib/tests/test_transport_implementations.py", line 69, in get_transport_test_permutations
    % module.__name__)
AssertionError: transport module bzrlib.plugins.auth.auth_remote doesn't provide get_test_permutations()

bzr 1.16 on python 2.6.2 (darwin)
arguments: ['/Library/Frameworks/Python.framework/Versions/2.6/bin/bzr', 'selftest']
encoding: 'UTF-8', fsenc: 'utf-8', lang: 'en_US.UTF-8'
plugins:
  auth /Users/tony/.bazaar/plugins/auth [unknown]
  automv /Users/tony/.bazaar/plugins/automv [unknown]
  bzrtools /Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/bzrlib/plugins/bzrtools [1.16]
  cvsps /Users/tony/.bazaar/plugins/cvsps [unknown]
  email /Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/bzrlib/plugins/email [unknown]
  extmerge /Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/bzrlib/plugins/extmerge [unknown]
  fastimport /Users/tony/.bazaar/plugins/fastimport [0.8dev]
  interactive /Users/tony/.bazaar/plugins/interactive [1.2]
  launchpad /Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/bzrlib/plugins/launchpad [1.16]
  log_rss /Users/tony/.bazaar/plugins/log_rss [0.1dev]
  loom /Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/bzrlib/plugins/loom [1.4dev]
  netrc_credential_store /Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/bzrlib/plugins/netrc_credential_store [1.16]
  notification /Users/tony/.bazaar/plugins/notification [unknown]
  plugin_info /Users/tony/.bazaar/plugins/plugin_info [1.3dev]
  push_and_update /Users/tony/.bazaar/plugins/push_and_update [0.2dev]
  qbzr /Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/bzrlib/plugins/qbzr [0.9.9]
  search /Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/bzrlib/plugins/search [1.7dev]
  stats /Users/tony/.bazaar/plugins/stats [unknown]
  svn /Users/tony/.bazaar/plugins/svn [0.5.3]
  upload /Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/bzrlib/plugins/upload [1.0dev]
  usertest /Users/tony/.bazaar/plugins/usertest [unknown]
  xmloutput /Users/tony/.bazaar/plugins/xmloutput [0.8.4]
*** 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.

As matt says:
   "bzr plugins" might not list plugins that couldn't be loaded.

   It's a pain, but you can search for the traceback in .bzr.log (use "bzr
   version" to find its location). It should show bzr-rebase's path.

Additionally, you can try 'bzr selftest --no-plugins'

If you can't debug the issue from the two tips above, please file a bug: https://bugs.launchpad.net/bzr/+filebug

I don't think continuing the discussion here is:
- the right use of the Answers fearture,
- the best way to help you solve your problem (bugs are far better)