I get a reproducible crash under Linux when running the test cases of
lxml's trunk in Py3b2. As usual with these things, it's not reproducible
when running the crashing test by itself, only when it hits the test
during the usual test run (which makes it look like somethings's leaking
between tests here).

Here is what gdb gives me so far.

----------------------
[...]
test_namespace_lookup (lxml.tests.test_classlookup.ClassLookupTestCase)
... ok
test_parser_based_lookup
(lxml.tests.test_classlookup.ClassLookupTestCase) ... ok
Doctest: test_css_select.txt ...
Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread -1210656576 (LWP 938)]
0x080f3b89 in BaseException_str (self=0x8d747d4) at Objects/exceptions.c:88
88          switch (PyTuple_GET_SIZE(self->args)) {
(gdb) pyo self
object  : AttributeError<NULL>
type    : AttributeError
refcount: 10
address : 0x8d747d4
$1 = void
(gdb) print self->args
$2 = (PyObject *) 0x0
(gdb) bt
#0  0x080f3b89 in BaseException_str (self=0x8d747d4) at
Objects/exceptions.c:88
#1  0x0805b158 in PyObject_Str (v=0x8d747d4) at Objects/object.c:414
#2  0x0807951a in unicode_new (type=0x81523a0, args=0x8de77ac, kwds=0x0)
at Objects/unicodeobject.c:9247
#3  0x0806068d in type_call (type=0x81523a0, args=0x8de77ac, kwds=0x0)
at Objects/typeobject.c:636
#4  0x080d83a9 in PyObject_Call (func=0x81523a0, arg=0x8de77ac, kw=0x0)
at Objects/abstract.c:2178
#5  0x0808de50 in PyEval_EvalFrameEx (f=0x8e2d07c, throwflag=0) at
Python/ceval.c:3606
#6  0x0808fccd in PyEval_EvalFrameEx (f=0x8e2bf14, throwflag=0) at
Python/ceval.c:3481
#7  0x0808fccd in PyEval_EvalFrameEx (f=0x8e2cef4, throwflag=0) at
Python/ceval.c:3481
#8  0x0808fccd in PyEval_EvalFrameEx (f=0x8e2d4ec, throwflag=0) at
Python/ceval.c:3481
#9  0x08090f6b in PyEval_EvalCodeEx (co=0x8268458, globals=0xb7b772d4,
locals=0x0, args=0x8e2cea4, argcount=3, kws=0x8e2ceb0, kwcount=1,
defs=0x826c678, defcount=3, kwdefs=0x0,
    closure=0x0) at Python/ceval.c:2830
#10 0x0808f63e in PyEval_EvalFrameEx (f=0x8e2cd54, throwflag=0) at
Python/ceval.c:3491
#11 0x0808fccd in PyEval_EvalFrameEx (f=0x8dc37a4, throwflag=0) at
Python/ceval.c:3481
#12 0x0808fccd in PyEval_EvalFrameEx (f=0x8da1d7c, throwflag=0) at
Python/ceval.c:3481
#13 0x08090f6b in PyEval_EvalCodeEx (co=0x84b21d0, globals=0x847d79c,
locals=0x0, args=0x8da0dbc, argcount=2, kws=0x8da0dc4, kwcount=2,
defs=0x8591b78, defcount=3, kwdefs=0x0,
    closure=0x0) at Python/ceval.c:2830
#14 0x0808f63e in PyEval_EvalFrameEx (f=0x8da0c64, throwflag=0) at
Python/ceval.c:3491
#15 0x0808fccd in PyEval_EvalFrameEx (f=0x8da00ec, throwflag=0) at
Python/ceval.c:3481
#16 0x08090f6b in PyEval_EvalCodeEx (co=0xb7b80410, globals=0xb7b75824,
locals=0x0, args=0x8d6ee98, argcount=2, kws=0x89e8058, kwcount=0,
defs=0x826a518, defcount=1,
    kwdefs=0x0, closure=0x0) at Python/ceval.c:2830
#17 0x080fe855 in function_call (func=0x8267e6c, arg=0x8d6ee8c,
kw=0x8ccee84) at Objects/funcobject.c:628
#18 0x080d83a9 in PyObject_Call (func=0x8267e6c, arg=0x8d6ee8c,
kw=0x8ccee84) at Objects/abstract.c:2178
#19 0x0808d73f in PyEval_EvalFrameEx (f=0x8d9ff84, throwflag=0) at
Python/ceval.c:3694
#20 0x08090f6b in PyEval_EvalCodeEx (co=0xb7b80458, globals=0xb7b75824,
locals=0x0, args=0x8d7f078, argcount=2, kws=0x0, kwcount=0, defs=0x0,
defcount=0, kwdefs=0x0,
    closure=0x0) at Python/ceval.c:2830
[...]
(gdb) pyo 0x8268458
object  : <code object print_exception at 0x8268458, file
"/usr/local/python3.0b2/lib/python3.0/traceback.py", line 136>
type    : code
refcount: 2
address : 0x8268458
$3 = void
----------------------

When I call "pystack", gdb seems to lock up using 100% CPU, so I'm not
sure what else I can provide. The crash happening in a non-trivial
doctest makes is somewhat tricky to figure out what gets executed here.
At least, there is no anticipated AttributeError in the doctest, and it
doesn't seem to get raised when I run the test just by itself.