In rev 56476, martin.v.loewis enabled Py_USING_MEMORY_DEBUGGER by 
default in an huge commit:
   PEP 3123: Provide forward compatibility with Python 3.0, 
   while keeping backwards compatibility. Add Py_Refcnt, 
   Py_Type, Py_Size, and PyVarObject_HEAD_INIT.

I guess that's an error, and that Py_USING_MEMORY_DEBUGGER should be 
disabled by default.

Proposition to avoid such bug in future: create a configure option, 
like --with-memory-debugger. See for example my patch to configure.in 
(you will need to run autoconf && autoheader).

You are right that this was by mistake; here is a patch to revert that
mistake (revert.diff).

I'm marking the issue release-critical because of thsi patch; the other
patches (for adding a new configure option) clearly can't go into 2.6,
and might need to be deferred to 2.7.

@loewis: your patch (revert.diff) includes a change in configure.in 
about OpenBSD !?

Sorry about that - let me retry.

Looks good to me.

Committed as r66616 and r66617

Close the issue since it's commited in 2.6 and 3.0. My patch 
configure-memory-debugger.patch is useless, a developer can fix 
obmalloc.c.