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Running the following results in a Segmentation fault on Ubuntu 11.10 64-bit with both python and python3.
from itertools import *
c = count()
a,b = tee(c)
for i in range(10000000):
 next(a)
del(b)

tee() uses a linked-list of teedataobject. This list is destroyed by recursive calls to teedataobject_dealloc().
Extract of the gdb trace:
#5 0x08171a8e in teedataobject_clear (tdo=0xad21b394) at ./Modules/itertoolsmodule.c:412
#6 0x08171b39 in teedataobject_dealloc (tdo=0xad21b394) at ./Modules/itertoolsmodule.c:421
#7 0x0805ed3d in _Py_Dealloc (op=<itertools.tee_dataobject at remote 0xad21b394>) at Objects/object.c:1676
#8 0x08171b16 in teedataobject_clear (tdo=0xad21b274) at ./Modules/itertoolsmodule.c:413
#9 0x08171b39 in teedataobject_dealloc (tdo=0xad21b274) at ./Modules/itertoolsmodule.c:421
#10 0x0805ed3d in _Py_Dealloc (op=<itertools.tee_dataobject at remote 0xad21b274>) at Objects/object.c:1676
#11 0x08171b16 in teedataobject_clear (tdo=0xad21b154) at ./Modules/itertoolsmodule.c:413
#12 0x08171b39 in teedataobject_dealloc (tdo=0xad21b154) at ./Modules/itertoolsmodule.c:421
#13 0x0805ed3d in _Py_Dealloc (op=<itertools.tee_dataobject at remote 0xad21b154>) at Objects/object.c:1676
#14 0x08171b16 in teedataobject_clear (tdo=0xad21b034) at ./Modules/itertoolsmodule.c:413
#15 0x08171b39 in teedataobject_dealloc (tdo=0xad21b034) at ./Modules/itertoolsmodule.c:421
#16 0x0805ed3d in _Py_Dealloc (op=<itertools.tee_dataobject at remote 0xad21b034>) at Objects/object.c:1676
#17 0x08171b16 in teedataobject_clear (tdo=0xad292ed4) at ./Modules/itertoolsmodule.c:413
#18 0x08171b39 in teedataobject_dealloc (tdo=0xad292ed4) at ./Modules/itertoolsmodule.c:421
#19 0x0805ed3d in _Py_Dealloc (op=<itertools.tee_dataobject at remote 0xad292ed4>) at Objects/object.c:1676
#20 0x08171b16 in teedataobject_clear (tdo=0xad292db4) at ./Modules/itertoolsmodule.c:413
#21 0x08171b39 in teedataobject_dealloc (tdo=0xad292db4) at ./Modules/itertoolsmodule.c:421
#22 0x0805ed3d in _Py_Dealloc (op=<itertools.tee_dataobject at remote 0xad292db4>) at Objects/object.c:1676
#23 0x08171b16 in teedataobject_clear (tdo=0xad292c94) at ./Modules/itertoolsmodule.c:413

Confirmed on py3k, it doesn't seem to happen with smaller values.

Also, a check for NULL would not hurt in tee_next():
diff -r 1e0e821d2626 Modules/itertoolsmodule.c
--- a/Modules/itertoolsmodule.c	Fri Nov 04 22:17:45 2011 +0100
+++ b/Modules/itertoolsmodule.c	Tue Nov 22 17:24:42 2011 +0100
@@ -475,6 +475,8 @@
 
 if (to->index >= LINKCELLS) {
 link = teedataobject_jumplink(to->dataobj);
+ if (link == NULL)
+ return NULL;
 Py_DECREF(to->dataobj);
 to->dataobj = (teedataobject *)link;
 to->index = 0;

Here is a patch (tests included). Thank Pyry for report, Victor and Amaury for analysis.
Maybe I picked up the poor names for iterators? May be exhausted and unexhausted would be better? Feel free to correct me.

Serhiy, I only see a test in your patch, no actual modification to itertools.

Oh, I worked on it in a different directory. Fortunately I found a temporary copy and I do not have to write all code again. Sorry.

Please review.

Ping.

The patch replaces a
 Py_CLEAR(tdo->nextlink)
with a construct that does, basically, something like this several times:
 Py_DECREF(tdo->nextlink)
 tdo->nextlink
which is what leads to the issues that Py_CLEAR is supposed to prevent. Therefore I think this patch is not correct.

Good point. Here is updated patch.

If no one objects I will commit this next week.

New changeset f7e14a1af609 by Serhiy Storchaka in branch '3.2':
Issue #13454: Fix a crash when deleting an iterator created by itertools.tee()
http://hg.python.org/cpython/rev/f7e14a1af609
New changeset eff2a7346243 by Serhiy Storchaka in branch '3.3':
Issue #13454: Fix a crash when deleting an iterator created by itertools.tee()
http://hg.python.org/cpython/rev/eff2a7346243
New changeset 02d7a127fdfb by Serhiy Storchaka in branch 'default':
Issue #13454: Fix a crash when deleting an iterator created by itertools.tee()
http://hg.python.org/cpython/rev/02d7a127fdfb
New changeset 62f2d3f6015e by Serhiy Storchaka in branch '2.7':
Issue #13454: Fix a crash when deleting an iterator created by itertools.tee()
http://hg.python.org/cpython/rev/62f2d3f6015e 

New changeset 4ee8d38398d4 by Serhiy Storchaka in branch '2.7':
Optimize the test for issue #13454.
http://hg.python.org/cpython/rev/4ee8d38398d4
New changeset d391b2849a51 by Serhiy Storchaka in branch '3.2':
Optimize the test for issue #13454.
http://hg.python.org/cpython/rev/d391b2849a51
New changeset 2258b4d89c9f by Serhiy Storchaka in branch '3.3':
Optimize the test for issue #13454.
http://hg.python.org/cpython/rev/2258b4d89c9f
New changeset 1f57fb5e1e8e by Serhiy Storchaka in branch 'default':
Optimize the test for issue #13454.
http://hg.python.org/cpython/rev/1f57fb5e1e8e