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From obfsproxy/common/aes.py:

        self.ctr = Counter.new(128, initial_value=long(iv.encode('hex'), 16))

From the Crypto.Util.Counter docstring:

allow_wraparound : boolean
If *True*, the counter will automatically restart from zero after
reaching the maximum value (``2**nbits-1``).
If *False* (default), the object will raise an *OverflowError*.

The docs on the pycrypto web page are incorrect (https://github.com/dlitz/pycrypto/commit/f73d0a11697e2be0a3a1819fdb91a5dadc4e6af8) and haven't been regenerated yet.

The obfs2/3 protocols uses a initial value derived from the UniformDH handshake, allow_wraparound=True should be passed to the constructor here to avoid mysterious (though extremely unlikely) connection failures.