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# Breakpoint 1, PyUnicodeEncodeError_GetEnd (exc=<X at remote 0x405730e4>, end=0xbf9e8f7c) at Objects/exceptions.c:1643
# 1643 PyObject *obj = get_unicode(((PyUnicodeErrorObject *)exc)->object,
# (gdb) s
# get_unicode (attr=<unknown at remote 0x8c6a120>, name=0x82765ea "object") at Objects/exceptions.c:1516
# 1516 if (!attr) {
# (gdb) print *attr
# 4ドル = {_ob_next = 0xfefefefe, _ob_prev = 0xfefefefe, ob_refcnt = -16843010, ob_type = 0xfefefefe}
# (gdb) c
# Continuing.
# 
# Program received signal SIGSEGV, Segmentation fault.
# 0x080bc7d9 in get_unicode (attr=<unknown at remote 0x8cbe250>, name=0x82765ea "object") at Objects/exceptions.c:1521
# 1521 if (!PyUnicode_Check(attr)) {
#
# Type confusion. IsInstance check is ineffective because of custom 
# __getattribute__ method. Contents of string instance is interpreted as
# an exception object.

Here is simpler reproducer:
import codecs
class X(str):
 __class__ = UnicodeEncodeError
codecs.ignore_errors(X())
The problem is that PyObject_IsInstance() is fooled by custom __class__, but then builtin error handlers handle error object as having UnicodeEncodeError layout, while it doesn't.
Here is a patch that fixes the issue by using PyObject_IsSubclass() of exc->ob_type instead of PyObject_IsInstance().

The patch does indeed fix the segmentation fault. However the exception message looks confusing:
 TypeError: don't know how to handle UnicodeEncodeError in error callback

Here is a patch that makes error message consistent with type checking.

Looks much better. However shouldn't:
 exc->ob_type->tp_name
be:
 Py_TYPE(exc)->tp_name
(although there are still many spots in the source that still use ob_type->tp_name)

Py_TYPE() is necessary when the argument is not of type PyObject* (e.g. PyUnicodeObject*).

Also fixed handling errors of PyObject_IsSubclass() (issue24115) in the _codecs module.

New changeset 547bc11e3357 by Serhiy Storchaka in branch '2.7':
Issue #24102: Fixed exception type checking in standard error handlers.
https://hg.python.org/cpython/rev/547bc11e3357
New changeset 68eaa9409818 by Serhiy Storchaka in branch '3.4':
Issue #24102: Fixed exception type checking in standard error handlers.
https://hg.python.org/cpython/rev/68eaa9409818
New changeset 510819e5855e by Serhiy Storchaka in branch 'default':
Issue #24102: Fixed exception type checking in standard error handlers.
https://hg.python.org/cpython/rev/510819e5855e 

Greg Ewing suggested to use PyObject_TypeCheck (http://permalink.gmane.org/gmane.comp.python.devel/153216).