The following output is very suspect: the total number of references
decreases!

Python 3.0b3+ (py3k, Aug 24 2008, 21:56:40) [MSC v.1500 32 bit (Intel)]
on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> f(1=1)
  File "<stdin>", line 1
SyntaxError: keyword can't be an expression
[42055 refs]
>>> f(1=1)
  File "<stdin>", line 1
SyntaxError: keyword can't be an expression
[42054 refs]
>>> f(1=1)
  File "<stdin>", line 1
SyntaxError: keyword can't be an expression
[42053 refs]
>>> f(1=1)
  File "<stdin>", line 1
SyntaxError: keyword can't be an expression
[42052 refs]
>>> f(1=1)
  File "<stdin>", line 1
SyntaxError: keyword can't be an expression
[42051 refs]
>>> f(1=1)
  File "<stdin>", line 1
SyntaxError: keyword can't be an expression
[42050 refs]

After several hundred statements, I got:
Fatal Python error: deallocating None

The culprit is Python/pythonrun.c:1245:

    tb = tb ? tb : Py_None;

Py_None needs to be INCREF'd in the else case.

Agreed. PyErr_ExceptionMatches returns owned references, so the tb
variable has to remain an owned reference.
Patch attached, please review.

The patch looks fine to me.

Benjamin's reviewed it, so please commit it.  Is there a test for this
crasher?

Committed in r66041. Thanks everyone.