Steps to reproduce:
[giddens@quills 501 ~]$ mkdir reproduce
[giddens@quills 502 ~]$ cd !$
cd reproduce
[giddens@quills 503 reproduce]$ svnadmin create svn-repository
[giddens@quills 504 reproduce]$ svn co file://$PWD/svn-repository svn-sandbox
Checked out revision 0.
[giddens@quills 505 reproduce]$ (cd svn-sandbox && echo 'initial commit' > file && svn add file && svn ci -m 'initial commit')
A         file
Adding         file
Transmitting file data .
Committed revision 1.
[giddens@quills 506 reproduce]$ bzr init-repo bzr-repository
Shared repository with trees (format: 2a)
Location:
  shared repository: bzr-repository
[giddens@quills 507 reproduce]$ (cd bzr-repository && bzr branch file://$PWD/../svn-repository)
Initialising Subversion metadata cache in /home/giddens/.bazaar/svn-cache/8416993f-88bb-4b5e-bf51-df751838e050.
Branched 2 revision(s).                                                                                 
[giddens@quills 508 reproduce]$ mv bzr-repository/{svn-repository,trunk}
[giddens@quills 509 reproduce]$ (cd bzr-repository/trunk && echo 'change from trunk' >> file && bzr ci -m 'change from trunk')
Committing to: /home/giddens/reproduce/bzr-repository/trunk/                                            
modified file
Committed revision 3.                                                                                   
[giddens@quills 510 reproduce]$ (cd bzr-repository && bzr branch trunk feature)
Branched 3 revision(s).                                                                                 
[giddens@quills 511 reproduce]$ (cd bzr-repository/feature && echo 'change from feature' >> file && bzr ci -m 'change from feature')
Committing to: /home/giddens/reproduce/bzr-repository/feature/                                          
modified file
Committed revision 4.                                                                                   
[giddens@quills 512 reproduce]$ (cd bzr-repository/trunk && bzr merge ../feature && bzr ci -m 'merged feature branch')
 M  file                                                                                                
All changes applied successfully.                                                                       
Committing to: /home/giddens/reproduce/bzr-repository/trunk/                                            
modified file
Committed revision 4.                                                                                   
[giddens@quills 513 reproduce]$ (cd bzr-repository/trunk && echo 'subsequent change in trunk' >> file && bzr ci -m 'subsequent change in trunk' && bzr push file://$PWD/../../svn-repository)
Committing to: /home/giddens/reproduce/bzr-repository/trunk/                                            
modified file
Committed revision 5.
Pushed up to revision 5.
[giddens@quills 515 reproduce]$ bzr init-repo unrelated-repository
Shared repository with trees (format: 2a)
Location:
  shared repository: unrelated-repository
[giddens@quills 517 reproduce]$ (cd unrelated-repository/ && bzr branch file://$PWD/../svn-repository trunk-two)
Branched 5 revision(s).
[giddens@quills 519 reproduce]$ (cd unrelated-repository/trunk-two  && bzr diff -c 4)
=== modified file 'file'
bzr: ERROR: bzrlib.errors.NoSuchRevision: CHKInventoryRepository('file:///home/giddens/reproduce/unrelated-repository/.bzr/repository/') has no revision giddens@quills-20091021034530-1414x7xj7ioy7q85

*** 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
    attaching the crash file
        /home/giddens/.cache/crash/bzr-20091021035209-14702.crash
    and including a description of the problem.

    The crash file is plain text and you can inspect or edit it to remove
    private information.
[giddens@quills 520 reproduce]$