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The script below shows that the error handler is always called with the same error object. The 'start', 'end', and 'reason' properties are correctly updated, but the 'args' is always the same and holds the values used for the first call.
It's a bit weird that error.args[2] is not equal to error.start, for example. All versions are affected: 2.7, 3.2, 3.3.
And by the way, I could not find where these are attributes documented.
def custom_handler(error):
 print(error.args,
 (error.start, error.end, error.reason))
 return b'?'.decode(), error.end
import codecs
codecs.register_error('custom', custom_handler)
b'\x80\xd0'.decode('utf-8', 'custom')

See this ancient posting about this problem:
 http://mail.python.org/pipermail/python-dev/2002-August/027661.html
(see point 4.). So I guess somebody did finally complain! ;)
The error attributes are documented in PEP 293. The existence of the attributes is documented in Doc/c-api/exceptions.rst, but not their meaning.

Codec encoders reuse the same exception object for speed, but set some attributes (start, end and reason). Recreate the args tuple each time that a attribute is set. UnicodeEncodeError and UnicodeDecodeError should maybe override args getter to create a new tuple at each call.

For reference, this behavior was from beginning, since implementing PEP 293 in issue432401.