Frank: on the tahoe-dev mailing list (https://tahoe-lafs.org/pipermail/tahoe-dev/2013-August/008664.html), you posted this stack trace:

allmydata.test.test_client
  Basic
    test_create_drop_uploader ... Traceback (most recent call last):
  File "/Users/Frank/allmydata-tahoe-1.10.0/support/lib/python2.7/site-packages/mock-1.0.1-py2.7.egg/mock.py", line 1201, in patched
    return func(*args, **keywargs)
  File "/Users/Frank/allmydata-tahoe-1.10.0/src/allmydata/test/test_client.py", line 235, in test_create_drop_uploader
    self.failUnlessRaises(MissingConfigEntry, client.Client, basedir1)
  File "/Users/Frank/allmydata-tahoe-1.10.0/support/lib/python2.7/site-packages/Twisted-13.0.0-py2.7-macosx-10.5-i386.egg/twisted/trial/_synctest.py", line 335, in assertRaises
    failure.Failure().getTraceback()))
twisted.trial.unittest.FailTest: <type 'exceptions.UnicodeDecodeError'> raised instead of MissingConfigEntry:
 Traceback (most recent call last):
  File "/Users/Frank/allmydata-tahoe-1.10.0/support/lib/python2.7/site-packages/Twisted-13.0.0-py2.7-macosx-10.5-i386.egg/twisted/internet/defer.py", line 137, in maybeDeferred
    result = f(*args, **kw)
  File "/Users/Frank/allmydata-tahoe-1.10.0/support/lib/python2.7/site-packages/Twisted-13.0.0-py2.7-macosx-10.5-i386.egg/twisted/internet/_utilspy3.py", line 37, in runWithWarningsSuppressed
    result = f(*a, **kw)
  File "/Users/Frank/allmydata-tahoe-1.10.0/support/lib/python2.7/site-packages/mock-1.0.1-py2.7.egg/mock.py", line 1201, in patched
    return func(*args, **keywargs)
  File "/Users/Frank/allmydata-tahoe-1.10.0/src/allmydata/test/test_client.py", line 235, in test_create_drop_uploader
    self.failUnlessRaises(MissingConfigEntry, client.Client, basedir1)
--- <exception caught here> ---
  File "/Users/Frank/allmydata-tahoe-1.10.0/support/lib/python2.7/site-packages/Twisted-13.0.0-py2.7-macosx-10.5-i386.egg/twisted/trial/_synctest.py", line 328, in assertRaises
    result = f(*args, **kwargs)
  File "/Users/Frank/allmydata-tahoe-1.10.0/src/allmydata/client.py", line 130, in __init__
    node.Node.__init__(self, basedir)
  File "/Users/Frank/allmydata-tahoe-1.10.0/src/allmydata/node.py", line 82, in __init__
    self.create_tub()
  File "/Users/Frank/allmydata-tahoe-1.10.0/src/allmydata/node.py", line 174, in create_tub
    self.tub = Tub(certFile=certfile)
  File "/Users/Frank/allmydata-tahoe-1.10.0/support/lib/python2.7/site-packages/foolscap-0.6.4-py2.7.egg/foolscap/pb.py", line 240, in __init__
    self.setupEncryptionFile(certFile)
  File "/Users/Frank/allmydata-tahoe-1.10.0/support/lib/python2.7/site-packages/foolscap-0.6.4-py2.7.egg/foolscap/pb.py", line 252, in setupEncryptionFile
    self.setupEncryption(certData)
  File "/Users/Frank/allmydata-tahoe-1.10.0/support/lib/python2.7/site-packages/foolscap-0.6.4-py2.7.egg/foolscap/pb.py", line 267, in setupEncryption
    cert = self.createCertificate()
  File "/Users/Frank/allmydata-tahoe-1.10.0/support/lib/python2.7/site-packages/foolscap-0.6.4-py2.7.egg/foolscap/pb.py", line 476, in createCertificate
    132)
  File "/Users/Frank/allmydata-tahoe-1.10.0/support/lib/python2.7/site-packages/Twisted-13.0.0-py2.7-macosx-10.5-i386.egg/twisted/internet/_sslverify.py", line 577, in signCertificateRequest
    hlreq = CertificateRequest.load(requestData, requestFormat)
  File "/Users/Frank/allmydata-tahoe-1.10.0/support/lib/python2.7/site-packages/Twisted-13.0.0-py2.7-macosx-10.5-i386.egg/twisted/internet/_sslverify.py", line 348, in load
    dn._copyFrom(req.get_subject())
  File "/Users/Frank/allmydata-tahoe-1.10.0/support/lib/python2.7/site-packages/Twisted-13.0.0-py2.7-macosx-10.5-i386.egg/twisted/internet/_sslverify.py", line 101, in _copyFrom
    value = getattr(x509name, name, None)
exceptions.UnicodeDecodeError: 'utf8' codec can't decode byte 0xfd in position 0: invalid start byte

Here is a view of (I think) the Twisted-13.0.0 source code: https://github.com/twisted/twisted/blob/releases/release-13.0.0-6353/twisted/internet/_sslverify.py

So, _sslverify.py line 101 is ... Huh, that's weird: why does it try to use 'utf8' to decode a bytestring? What about "value = getattr(x509name, name, None)" causes a bytestring to get decoded into a unicode object, and why does it attempt to do so using utf8 instead of the default codec of 'ascii'?